Hawzah News Agency – If in the middle of a day in the month of Ramadan a sick person gets well before ẓuhr and until that time he did not do anything that invalidates a fast, then based on obligatory precaution he must make the intention to fast and keep the fast on that day; and in the event that he gets better after ẓuhr, it is not obligatory on him to fast on that day but he must keep a qaḍāʾ fast for it.

Source: https://www.sistani.org/english/book/48/