If fasting will be too difficult for the one who is sick, it is better for him not to fast, and to make up the days that he did not fast later on. It is not mustahabb for him to fast if it is too difficult.
The evidence for that is as follows:
1 – Ahmad (5832) narrated that Ibn ‘Umar said: The Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) said: “Allaah loves His concessions to be accepted just as He hates for acts of disobedience to be committed.” Classed as saheeh by al-Albaani in Irwa’ al-Ghaleel, 564.
2 – al-Bukhaari (6786) and Muslim (2327) narrated that ‘Aa’ishah (may Allaah be pleased with her) said: The Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) was never given the choice between two things but he would choose the easier of them, unless it was a sin. If it was a sin he would be the furthest away from it.
Al-Nawawi (may Allaah have mercy on him) said:
This shows that it is mustahabb to choose the easier and gentler option, so long as it is not haraam or makrooh.
Rather it is makrooh for one who is sick to fast if fasting is too difficult for him. His fasting may be haraam if there is the fear that he may be harmed because of fasting.
Al-Qurtubi (may Allaah have mercy on him) said: (2/276):
In the case of one who is sick, two scenarios may apply:
1 – He is not able to fast at all, so he has to break his fast and it is obligatory for him not to fast.
2 – If he is able to fast but that will cause him harm and be difficult for him. In this case it is mustahabb for him to break his fast and not to fast; in this case only an ignorant person would fast.
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni, 4/404:
If a sick person is able to put up with fasting and does so, then he has done something which is makrooh, because of the harm that results from that and because he has neglected the concession granted by Allaah.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in al-Sharh al-Mumti’ (6/352):
From this we realize the mistake made by the mujaahideen and sick people for whom fasting is difficult and may even cause them harm, but they refuse to break the fast. We said that they are making a mistake when they do not accept the kindness of Allaah and the concession He has granted, and they are harming themselves. Allaah says (interpretation of the meaning):
“And do not kill yourselves”
See also Question no. 1319.
And Allaah knows best.