Nasai

It is as if, after ten months, in Sha'ban she would make up for the missed fasts of the previous Ramadan. From this hadith, it is understood that making up (qada) for obligatory fasts is not immediately necessary; it is possible to make up the missed fasts at any time during the year. However, it is better to strive to make up the missed fasts as soon as possible, for who knows about illness or death? Furthermore, it is also established that a menstruating woman is not excused from making up missed fasts; rather, she must observe those fasts later in any case. The reason for the delay in making up the fasts by Aisha (radi Allahu anha) is also reported: it was lest the Prophet (sallallahu alayhi wa sallam) might feel the need for her and she would be fasting. In Sha'ban, the Messenger of Allah (sallallahu alayhi wa sallam) would also often be fasting. (For further details, see Hadith: 2180)