The Messenger of Allah ﷺ used to fast until we would say he never broke his fast, and he would go without fasting until we would say he never fasted. He never fasted a complete month, since he arrived in Medina, except Ramadan.
حدثنا يحيى عن شعبة حدثنا أبو بشر عن سعيد بن جبير
كان رسول الله ﷺ يصوم حتى نقول لا يفطر ويفطر حتى نقول لا يصوم وما صام شهرا تاما منذ قدم المدينة إلا رمضان
Its isnad is sahih. Yahya is Ibn Saeed Qattan. In (ح), it says "Yahya, from Saeed, Abu Bishr narrated to us," and in (ك) it says "Yahya bin Saeed, Abu Bishr narrated to us." Both are mistakes, for Qattan never met Abu Bishr Jafar bin Abu Wahshiya. Yahya was born in the year 120, and Abu Bishr died in the year 123 or 125. None among the narrators from Abu Bishr is named Saeed.Moreover, the hadith is the hadith of Shuba from Abu Bishr. Tayalisi narrated it (2626) from Shuba, and Muslim narrated it (1:318) through the route of Ghundar from Shuba. He also narrated it through the route of Uthman bin Hakeem Ansari from Saeed bin Jubayr, and he, along with Bukhari (45:188), narrated it through the route of Abu Awana from Abu Bishr. It will also come later at 2046, 2151, 2450, 2737, 2949, and 3011.