I never used to make up the days I owed from Ramadan except in Shaban, until the Messenger of Allah ﷺ passed away.
حدثنا عفان حدثنا أبو عوانة حدثنا إسماعيل السدي عن عبد الله البهي
ما كنت أقضي ما يكون علي من رمضان إلا في شعبان حتى توفي رسول الله ﷺ
Its isnad is sahih, though there is some discussion regarding Suddi. Bukhari related the hadith 4/189, no. 1900 (Fath), Muslim 2/802, no. 1146, Abu Dawud 2/315, no. 2399, Nasai 4/191, no. 239, and Ibn Majah 1/533, no. 1669, all of them in the chapter on fasting.