(34) بَاب صِيَامِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي غَيْرِ رَمَضَانَ وَاسْتِحْبَابِ أَنْ لَا يُخْلِيَ شَهْرًا عَنْ صَوْمٍ
Abdullah b. Shaqiq reported:
I said to'A'isha (Allah be pleased with her): Did the Messenger of Allah (ﷺ) fast for a full month besides Ramadan? She said: I do not know of any month in which he fasted throughout, but that of the month of Ramadan and (the month) in which he did not fast at all, till he ran the course of his life.
Reference: Sahih Muslim 1156 a
In-book reference: Book 13, Hadith 223
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2576