Sunan an-Nasa'i 2178
سنن النسائي#2178SahihIt was narrated that Aishah said:"One of us (women) would miss some fasts in Ramadan and she would not be able to make it up until Shaban began, and the Messenger of Allah did not fast in any month as he fasted in Shaban; he used to fast all of it, except a little, he used to fast all of it
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih Muslim
In-book reference: 22:89