(70) باب صَوْمِ النَّبِيِّ صلى الله عليه وسلم - بِأَبِي هُوَ وَأُمِّي - وَذِكْرِ اخْتِلاَفِ النَّاقِلِينَ لِلْخَبَرِ فِي ذَلِكَ
It was narrated that Hafsah said:
"The Messenger of Allah used to fast Thursday and Monday of each month, and Monday of the following week."
Reference: Sunan an-Nasa'i 2366
In-book reference: Book 22, Hadith 277
English translation: Vol. 3, Book 22, Hadith 2368