Taken from his Explanation of Riyad al-Salihin

Allah says,

Believers, fasting is prescribed upon you as it was prescribed upon those before you, so that you may be from amongst the righteous, for a fixed number of days. But if you are ill or travelling, then an equal number of other days. As for those who can only fast with extreme difficulty, then the feeding of the poor. But whoever does good of his own accord, it is better for him. But fasting is better for you if you only knew. The month of Ramadan in which the Qur’an was revealed as guidance for humanity, and as clear signs giving guidance and as a criterion. So whoever witnesses the month should fast in it. But whoever is sick or travelling, then an equal number from other days. [Surah al-Baqarah, 2:183-5]

The fast of Ramadan is a worship of Allah, which comprises of abstaining from eating, drinking, and sexual intercourse from dawn until sunset. A person does not abstain from them for a customary thing or for a health reason, but does so for the sole purpose of worshipping Allah. Thus, he abstains from food, drink, sexual intercourse, or any other thing that would nullify the fast, doing so from dawn to sunset, from the beginning of Ramadan until its end. The fast of Ramadan is one of the pillars of Islam, and it is obligatory based on a unanimous agreement of the Muslims based on the Qur’an and the Sunnah.

Imam al-Nawawwi then mentions verses that show its obligation, such as the words of Allah,

“Believers, fasting is prescribed upon you as it was prescribed upon those before you so that you may be from amongst the righteous.”

Allah addresses the believers because the fast of Ramadan is from among the requirements of faith. Thus, fasting in Ramadan causes ones faith to become complete, while abandoning it would cause deficiency in his faith. The scholars have differed over whether abandoning it due to negligence and laziness causes a person to commit disbelief? The correct opinion is that it does not cause disbelief. The abandonment of any of the pillars of Islam by a person does cause him to commit disbelief except the abandonment of the testimonies of faith and salah.

The words of Allah,

Fasting is prescribed upon you

Means that it has been made obligatory.

As it was prescribed

Meaning, as it was made obligatory,

Upon those before you.

Allah mentions that it was made obligatory upon those before us, but He did not mention this in regards to salah. This is because fasting contains difficulties, fatigue, and abstinence from habitual things. It is not hidden that it becomes even more difficult upon the soul during the days that are hot and long. Thus, Allah mentions that He had also made it obligatory upon those before us in order to bring happiness us, because when a person knows that a thing that he has been asked to perform has also been asked of others, then it makes it easier for him to perform it. He also made mention of it in order to explain that He has perfected His blessings upon us, just as He completed His blessings on those who came before us.

Allah then says,

So that you may be from amongst the righteous.

So that you may be conscious of Allah, since fasting is a shield that protects you from sins and protects you from the Hellfire. This is because whoever observes the fast of Ramadan with faith and hoping for reward, then all his previous sins will be forgiven. The words, “So that you may be from amongst the righteous,” meaning, for the sake of piety, since this is the wisdom behind the obligation of fasting. The following words from the Prophet point to this,

“Whoever does not avoid lying and acting on it and ignorance, then Allah does not have any need for him to abstain from food and drink.”

This is because Allah does not want them to merely abstain from what they desire and are accustomed to, but He wants them to abstain from falsehood, acting upon it, and ignorance.

Allah then says,

For a fixed number of days

He said this to explain that the issue is not for a couple of months or years, but for only some days, and so it is not for a long time.

But if you are ill or travelling, then an equal number of other days.

It can mean that Allah is relating that it is only for a few and fixed days, or that for whomever the fast is difficult due to an illness or travel, then he should break his fast and make it up at other days.

As for those who can only fast with extreme difficulty

And this is in reference to those who are residents,

Then the feeding of the poor. But whoever does good of his own accord, it is better for him.

However, this was in the early days when Allah had mandated fasting, and so if a person was unable to fast, then he could simply feed the poor. If a person gave in charity, then that was better for him, and if he fasted, then that was better for you. Allah initially gave the people a choice between fasting or feeding the poor person for each day of fasting. However, He immediately specified the following,

But fasting is better for you if you only knew

Meaning, if you are people of knowledge and understanding. It is well known that fasting is more difficult than feeding the poor, and since it is more difficult, then it is better since performing a task that is more difficult, while following that which Allah has commanded, will incur a reward that is also greater. This is why we see that the one that lives farther from the mosque has a greater reward when walking to the mosque to attend the prayer.

However, this does not mean that a person should desire difficulty in worships that Allah has made easy, since would be extremism in the religion. But, if Allah has obligated a worship upon you, which is difficult for you, then know that the reward will be greater. But to purposely seek difficulty in a worship goes against the command. For example, some ignorant people perform wudu’ with cold water during the winter days, thinking that by doing so they will be raised in rank and their sins will be removed, since they are doing it while there is difficulty in doing so. However, We say to these brothers of ours that this is not what Allah’s Messenger intended; rather, what he intended is that if a person has to perform wudu’ with cold water during the days of winter, then his reward will be greater. However, he did not say that a person should seek out cold water. Therefore, if you are blessed with Allah with hot water, and you are able to perform wudu’ with it, then that would be better.

But if you are ill

And the scholars have classified illness into three types: The first type is an illness that has no hope of recovery, and is something that is continuous. In this case, there will be no fasting on the sick person, but he must feed a poor person for each day, since he is similar to an old person who is unable to fast and who has no hope of having his weakness removed.

The second type is an illness that will be further aggravated if a person fasts, or it may even cause his death, as he may not be able to go without water. In this case, fasting will be unlawful for him. This is based on the words of Allah,

Do not kill yourselves. Allah is certainly merciful to you. [Surah al-Nisa’, 4:29]

The third type is an illness that will cause difficult while fasting, but will not bring about a greater harm. Here, the best is not to fast and to make it up later. As for an illness that will not be affected by fasting in any way, such as an ailment in the eye or the teeth, or something similar, then it is not permissible to break the fast. This is because the wisdom behind the concession is removal of the difficult, and there is no absolute difficulty on such a person, and so it is not permissible for him to break the fast. The original foundation is obligation of fast during its time, and it can only be alleviated if there is a clear and plain evidence that permits the person to break the fast and then make it up later.

As for traveling, then fasting is also divided into three types during it. The first type that which will be harmed by fasting, and so it makes is very difficult for a person to journey during it. An example is traveling during hot days or long days, and a person knows that if he fasts, then it will bring about harm to him and cause him unbearable difficulty. For such a person, if he were to fast, then he would be sinning. The evidence for this is that the people were once journey with the Prophet and he was told that the people were finding it difficult to fast, and so he called for some water and drank it while people looked on, and this was after ‘asr. He did this so that the people would not have any blame in their hearts if they broke their fast. However, some Companions continued to fast, and when this was related to the Prophet, he said,

“They are disobedient; they are disobedient.”

He described them with disobedience because they did not accept the concession given by Allah, despite the situation being difficult for them.

The second type is when it is difficult but bearable for a person. In this case, it is disliked for him to fast, and it will not be considered as righteousness for him to fast. The proof for this is that the Prophet was once on a journey he saw a crowed shading a man. He asked, “What is this?” They replied, “He is fasting.” He said,

“It is not part of righteousness to fast on a journey.”

The third type is the one that is not affected by the fast absolutely, since the day may be short, or the weather is cool and so it does not cause the fasting person difficulty. Regarding such a person, the scholars differ over what is best for him, should he break the fast or should he continue to fast? The correct opinion is that is it better for him to fast. This is because following the sunnah of the Prophet under this scenario is greater, and this is something that is easier on those who are morally responsible. It is well-known that fasting along with the people is a lot easier than making it up alone, and because hastening to fulfill an obligation in the time that it falls in, which is the month of Ramadan, is better. On the basis of these points, fasting in this scenario would be best. Abu Darda related,

“We accompanied the Prophet on his journeys during hot days, so much so that a man would place his hand on his head due to the intensity of the heat, and none of us would be fasting except the Prophet and ‘Abdullah b. Rawahah.”

These are the rulings concerning fasting while travelling. A journey can be one of a general nature, such as journeying for umrah or other thing, or it can be one of a continuous nature, such as driving a taxi or a truck, in which case they should break their fast even if their journey is continuous, since they have a country that they will return to. Thus, when they separate from their country, they are considered as travelers. If a person was to ask, “When should they fast [if they are travelling continuously]?” Then we say that they should fast during the winter days, or whenever it is easier for them. However, if they return to their country during Ramadan, then it is binding on them to begin the fasting since the ruling of travelling has been removed from them.