(70) باب صَوْمِ النَّبِيِّ صلى الله عليه وسلم - بِأَبِي هُوَ وَأُمِّي - وَذِكْرِ اخْتِلاَفِ النَّاقِلِينَ لِلْخَبَرِ فِي ذَلِكَ
It was narrated that 'Abdullah bin Masud said:
"The Messenger of Allah used to fast three days in the middle of every month, and he rarely did not fast on Friday."
Reference: Sunan an-Nasa'i 2368
In-book reference: Book 22, Hadith 279
English translation: Vol. 3, Book 22, Hadith 2370