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 1157 b

This hadith has been narrated on the authority of Abu Bishr with the same chain of transmitters (with a slight variation of words and these are), that he (the narrator) said:

" During any month continuously since he came to Medina."
وَحَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ وَأَبُو بَكْرِ بْنُ نَافِعٍ عَنْ غُنْدَرٍ عَنْ شُعْبَةَ عَنْ أَبِي بِشْرٍ بِهَذَا الْإِسْنَادِ وَقَالَ شَهْرًا مُتَتَابِعًا مُنْذُ قَدِمَ الْمَدِينَةَ
Reference : Sahih Muslim 1157b
In-book reference : Book 13, Hadith 231
USC-MSA web (English) reference : Book 6, Hadith 2584
  (deprecated numbering scheme)