Narrated Aishah: If I had some part of the fast of Ramadan to make up, I would not be able to atone for it except in Sha'ban.
Explanation & Benefits
Shaykh Umar Farooq Saeedi
Benefits and Issues:
➊ Due to being occupied in the service of the Messenger of Allah (sallallahu alayhi wa sallam), she did not get the opportunity to observe fasts, until Sha'ban would arrive and the Messenger of Allah (sallallahu alayhi wa sallam) would frequently fast in it, so she would also get the chance to make up the missed fasts.
➋ It is permissible to delay making up missed fasts with the confidence that there will be an opportunity to do so.
➌ Attending to the service of the husband is included among the wife's obligations.
➍ If Ramadan arrives and one has not yet made up missed fasts, then one should make them up after Ramadan. In this case, some of the Companions (radi Allahu anhum), the Followers (tabi'un), and others hold the opinion that along with making up the fasts, one should also feed a poor person for each missed day, while others say that nothing is required except making up the fasts.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2399