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

'A'isha (Allah be pleased with her) reported:

The Messenger of Allah (ﷺ) did not observe fast in any month of the year more than in the month of Sha'ban, and used to say: Do as many deeds as you are capable of doing, for Allah will not become weary (of giving you reward), but you would be tired (of doing good deeds) ; and he also said: The deed liked most by Allah is one to which the doer adheres constantly even if it is small.
حَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، أَخْبَرَنَا مُعَاذُ بْنُ هِشَامٍ، حَدَّثَنِي أَبِي، عَنْ يَحْيَى بْنِ، أَبِي كَثِيرٍ حَدَّثَنَا أَبُو سَلَمَةَ، عَنْ عَائِشَةَ، - رضى الله عنها - قَالَتْ لَمْ يَكُنْ رَسُولُ اللَّهِ صلى الله عليه وسلم فِي الشَّهْرِ مِنَ السَّنَةِ أَكْثَرَ صِيَامًا مِنْهُ فِي شَعْبَانَ وَكَانَ يَقُولُ ‏"‏ خُذُوا مِنَ الأَعْمَالِ مَا تُطِيقُونَ فَإِنَّ اللَّهَ لَنْ يَمَلَّ حَتَّى تَمَلُّوا ‏"‏ ‏.‏ وَكَانَ يَقُولُ ‏"‏ أَحَبُّ الْعَمَلِ إِلَى اللَّهِ مَا دَاوَمَ عَلَيْهِ صَاحِبُهُ وَإِنْ قَلَّ ‏"‏ ‏.‏
Reference : Sahih Muslim 782 c
In-book reference : Book 13, Hadith 229
USC-MSA web (English) reference : Book 6, Hadith 2582
  (deprecated numbering scheme)