If one does not fast some days during Ramadan, it is obligatory to make up these missed fasts before the next Ramadan arrives, regardless of whether these fasts were missed with a valid excuse (e.g. menstruation, travel, sickness, etc.) or without a valid excuse. If one does not make them up before the next Ramadan, one is sinful and must pay a “mudd” (a volumetric measure defined below) of food to someone poor (faqeer) or short of money (miskeen) in addition to making up the missed fasts (I`anatu’l-Talibin, 2.242; Tuhfat al-Muhtaj, 3.445-446).
Imam Daraqutni and Imam Bayhaqi (Allah be pleased with them) have related the following hadith from the Prophet (Allah bless him and give him peace):
Whoever lives to meet Ramadan, does not fast because of an illness, then regains his health and does not make up [the missed fasts] until another Ramadan should fast [the Ramadan] that he has reached, then make up what he owes, and then feed someone short of money [miskeen] for every day [he missed].
Both Imam Daraqutni and Imam Bayhaqi (Allah be pleased with them) said this hadith was weak (da`if), but Imam Ramli has mentioned that it has been narrated with sound (sahih) chains of transmission as a statement of a companion (a mawquf hadith). All this is also strengthened by the fact that six companions gave this fatwa and no one objected to them (Hashiyat al-Sharqawi, 1.413).