From Sahih Muslim (Hadith 2687):
Abu Salama reported:I heard 'A'isha (Allah be pleased with her) as saying: I had to complete some of the fasts of Ramadan, but I could not do it but during the month of Sha'ban due to my duties to the Messenger of Allah (ﷺ) or with the Messenger of Allah (ﷺ)
Reference: Muslim 2687