It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that the Messenger of Allah, peace and blessings be upon him, said: Whoever reaches the month of Ramadan while he still has fasts of the previous Ramadan pending to make up, then the fasts of this Ramadan will not be accepted from him. Similarly, whoever observes voluntary fasts while he still has obligatory fasts of Ramadan pending to make up, then these voluntary fasts will not be accepted until he makes up the obligatory ones.

It is narrated from Sayyida Aisha (may Allah be pleased with her), she says: My practice was that I used to make up the missed fasts of Ramadan in Sha'ban, until the Messenger of Allah (peace and blessings be upon him) passed away.