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 مُحَمَّدُ بْنُ بَشَّارٍ وَأَبُو بَكْرِ بْنُ نَافِعٍ عَنْ غُنْدَرٍ عَنْ شُعْبَةَ عَنْ أَبِي بِشْرٍ بِهَذَا الْإِسْنَادِ وَقَالَ شَهْرًا مُتَتَابِعًا مُنْذُ قَدِمَ الْمَدِينَةَ
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."
Reference : Sahih Muslim 1157bIn-book reference : Book 13, Hadith 231USC-MSA web (English) reference : Book 6, Hadith 2584