(34) بَاب صِيَامِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي غَيْرِ رَمَضَانَ وَاسْتِحْبَابِ أَنْ لَا يُخْلِيَ شَهْرًا عَنْ صَوْمٍ
Anas (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) used to observe fast till it was said that he had observed fast, he had observed fast (perhaps never to break it), and he did not fast till it was said that he had given up fast, he had given up fast (perhaps never to observe it).
Reference: Sahih Muslim 1158
In-book reference: Book 13, Hadith 234
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2586