Which is better for the sick person and the traveller: that they fast or that they break their fast?
We say: what is better is for them to do whichever is easier. If fasting causes harm, then fasting is unlawful, due to the saying of Allah, Most High: “And do not kill yourselves. Surely Allah is Most Merciful to you.” [al-Nisa’ 4:29] This verse indicates that whatever harms a person is prohibited.
If someone says: “This refers only to killing, not to harm in general,” the response is: Yes, that is the apparent meaning of the verse. However, ‘Amr ibn al-‘As, may Allah be pleased with him, cited it as evidence that harm is to be avoided, and the Prophet ﷺ approved of this. The Prophet ﷺ had dispatched him with a military detachment, and he became in a state of major ritual impurity. He therefore performed dry ablution and did not bathe. The Prophet ﷺ said to him, “Did you lead your companions in prayer while you were in a state of major ritual impurity?” He replied, “O Messenger of Allah, I remembered the saying of Allah, Most High: ‘And do not kill yourselves. Surely Allah is Most Merciful to you.’ The night was cold, so I performed dry ablution.” The Messenger of Allah ﷺ smiled (1), thereby approving his action. This shows that the verse includes a prohibition against killing oneself and against everything that causes harm.
Accordingly, we say: if fasting harms the sick person, then fasting is unlawful for him.
If someone says: “What is the criterion for harm?” We say: harm may be known through direct experience, and it may also be known by reports. As for direct experience, it is when the sick person himself feels that fasting harms him, aggravates his pain, delays his recovery, and the like.