It is narrated from Abu Salamah (may Allah have mercy on him) that he heard from Aisha (may Allah be pleased with her) who said: I used to have to make up the missed fasts of Ramadan, but I would not have the ability to do so before Sha'ban. (That is, I would make them up in Sha'ban.)
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 660
Hadith Takhrijاخرجه البخاري، الصوم، باب متى يقضى قضاء رمضان (1950) ۔ ومسلم، الصيام، باب جواز تأخیر قضاء رمضان مالم .... الخ (1146)۔