صَحِيحُ مُسْلِمٍ
Sahih Muslim 1156 a
The Book of Fasting
The Prophet's Fasts at times other than Ramadan; and it is recommended to ensure that no month is free of fasting
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا يَزِيدُ بْنُ زُرَيْعٍ، عَنْ سَعِيدٍ الْجُرَيْرِيِّ، عَنْ عَبْدِ اللَّهِ، بْنِ شَقِيقٍ قَالَ قُلْتُ لِعَائِشَةَ رضى الله عنها هَلْ كَانَ النَّبِيُّ صلى الله عليه وسلم يَصُومُ شَهْرًا مَعْلُومًا سِوَى رَمَضَانَ قَالَتْ وَاللَّهِ إِنْ صَامَ شَهْرًا مَعْلُومًا سِوَى رَمَضَانَ حَتَّى مَضَى لِوَجْهِهِ وَلاَ أَفْطَرَهُ حَتَّى يُصِيبَ مِنْهُ .
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.
Sahih Muslim, 1156 aSahihصحيح