(70) باب صَوْمِ النَّبِيِّ صلى الله عليه وسلم - بِأَبِي هُوَ وَأُمِّي - وَذِكْرِ اخْتِلاَفِ النَّاقِلِينَ لِلْخَبَرِ فِي ذَلِكَ
It was narrated that Umm Salamah said:
"The Messenger of Allah used to fast three days of every month: the Monday and Thursday of the first week and the Monday of the following week."
Reference: Sunan an-Nasa'i 2365
In-book reference: Book 22, Hadith 276
English translation: Vol. 3, Book 22, Hadith 2367