كتاب الصيام
13  
The Book of Fasting
(34)
Chapter: The Prophet's Fasts at times other than Ramadan; and it is recommended to ensure that no month is free of fasting
(34)
بَاب صِيَامِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي غَيْرِ رَمَضَانَ وَاسْتِحْبَابِ أَنْ لَا يُخْلِيَ شَهْرًا عَنْ صَوْمٍ
Sahih Muslim 1156 f

Abu Salama reported:

I asked 'A'isha (Allah be pleased with her) about the fasting of the Messenger of Allah (ﷺ). She said: He used to observe fast (at times so continuously) that we said: He has fasted (never to break), and he did not observe fast till we said: He has given up perhaps never to fast, and I never saw him observing (voluntary fasts) more in any other month than that of Sha'ban. (lt appeared as if) he observed fast throughout the whole of Sha'ban except a few (days).
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَعَمْرٌو النَّاقِدُ، جَمِيعًا عَنِ ابْنِ عُيَيْنَةَ، - قَالَ أَبُو بَكْرٍ حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، - عَنِ ابْنِ أَبِي لَبِيدٍ، عَنْ أَبِي سَلَمَةَ، قَالَ سَأَلْتُ عَائِشَةَ - رضى الله عنها - عَنْ صِيَامِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَتْ كَانَ يَصُومُ حَتَّى نَقُولَ قَدْ صَامَ ‏.‏ وَيُفْطِرُ حَتَّى نَقُولَ قَدْ أَفْطَرَ ‏.‏ وَلَمْ أَرَهُ صَائِمًا مِنْ شَهْرٍ قَطُّ أَكْثَرَ مِنْ صِيَامِهِ مِنْ شَعْبَانَ كَانَ يَصُومُ شَعْبَانَ كُلَّهُ كَانَ يَصُومُ شَعْبَانَ إِلاَّ قَلِيلاً ‏.‏
Reference : Sahih Muslim 1156f
In-book reference : Book 13, Hadith 228
USC-MSA web (English) reference : Book 6, Hadith 2581
  (deprecated numbering scheme)