Maliks Muwatta Book 18, Hadith Number 41.
Yahya said that he heard Malik say, “What I have heard from the people of knowledge is that if a man succumbs to an illness which makes fasting very difficult for him and exhausts him and wears him out, he can break his fast. This is the same as with a sick man in the prayer, who finds standing to be too difficult and exhausting, (and Allah knows better than the slave that it is an excuse for him and that it really cannot be described). If the man is in such a condition he prays sitting, and the deen of Allah is ease.
Allah has permitted a traveller to break the fast when travelling, and he has more strength for fasting than a sick man. Allah, the Exalted, says in His book, ‘Whoever among you is ill or on a journey (must fast) a number of other days,’ and Allah has thus permitted a traveller to break his fast when on a journey, and he is more capable of fasting than a sick man.