(81) باب صَوْمِ ثَلاَثَةِ أَيَّامٍ مِنَ الشَّهْرِ
It was narrated that Abu Hurairah said:
"The Messenger of Allah enjoyed three things upon me: To sleep after praying Witr, to perform Ghusal on Friday, and to fast three days of each month." (Sahih).
Reference: Sunan an-Nasa'i 2405
In-book reference: Book 22, Hadith 316
English translation: Vol. 3, Book 22, Hadith 2407