Praise be to Allaah.
If a Muslim does not fast in Ramadaan for a shar’i reason, he has to make up that fast after the reason no longer applies. He must hasten to do this as soon as possible. Allaah says (interpretation of the meaning):
“…but if any of you is ill or on a journey, the same number (should be made up) from other days…”
The intention of fasting should be made from the night before. The site of the intention is the heart, and it means wanting to do that action and resolving to do it. Thus the intention is achieved. There is no need to utter any words; the intention should be in the heart. The intention should be to do that deed for the sake of Allaah, so that the action will be sincere. The Prophet (peace and blessings of Allaah be upon him) said:
“Actions are but by intentions…”