Sahih Muslim
Sahih Muslim 1156 d
The Prophet's Fasts at times other than Ramadan; and it is recommended to ensure that no month is free of fasting
Arabic hadith text
وَحَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا حَمَّادٌ، عَنْ أَيُّوبَ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ، قَالَ سَأَلْتُ عَائِشَةَ - رضى الله عنها - بِمِثْلِهِ وَلَمْ يَذْكُرْ فِي الإِسْنَادِ هِشَامًا وَلاَ مُحَمَّدًا
Translation
A hadith like this has been narrated on the authority of Abdullah b. Shaqiq but in the chain of transmitters no mention is made of Hisham and Muhammad.