The Book of Fasting
13/34. Chapter:
The Prophet's Fasts at times other than Ramadan; and it is recommended to ensure that no month is free of fasting
Sahih Muslim
وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا كَهْمَسٌ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ، قَالَ قُلْتُ لِعَائِشَةَ - رضى الله عنها - أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصُومُ شَهْرًا كُلَّهُ قَالَتْ مَا عَلِمْتُهُ صَامَ شَهْرًا كُلَّهُ إِلاَّ رَمَضَانَ وَلاَ أَفْطَرَهُ كُلَّهُ حَتَّى يَصُومَ مِنْهُ حَتَّى مَضَى لِسَبِيلِهِ صلى الله عليه وسلم .
'Abdullah b. Shaqiq reported: I said to 'A'isha (Allah be pleased with her): Did the Messenger of Allah (ﷺ) observe fast during a month? She said, I do not know of any month in which he fasted throughout except Ramadan and (the month) in which he did not fast at all till he ran the course of his life. May peace be upon him.
Reference : Sahih Muslim 1156bIn-book reference : Book 13, Hadith 224USC-MSA web (English) reference : Book 6, Hadith 2577