A person who has an illness that is not expected to be cured and prevents them from fasting must feed a needy person one mudd (600 grams) of food (such as wheat or rice) for each missed day instead of fasting.

Allah Almighty says {what means}: "and [in such cases] it is incumbent upon those who can afford it to make sacrifice by feeding a needy person." [Al-Baqarah/184].