(34) بَاب صِيَامِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي غَيْرِ رَمَضَانَ وَاسْتِحْبَابِ أَنْ لَا يُخْلِيَ شَهْرًا عَنْ صَوْمٍ
Ibn Abbas (Allah be pleased with both of them) reported:
The Messenger of Allah (ﷺ) did not fast throughout any month except during ramadan. And when he observed fast (he fasted so continuously) that one would say that he would not break (them) and when he Abandoned, he abandoned (so continuously) that one would say: By Allah, perhaps he would never fast.
Reference: Sahih Muslim 1157 a
In-book reference: Book 13, Hadith 230
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2583