(81) باب صَوْمِ ثَلاَثَةِ أَيَّامٍ مِنَ الشَّهْرِ
It was narrated that Abu Dharr said:
"My beloved Prophet advised me to do three things which I will never give up, if Allah wills. He advised me to pray Duha, to pray Witr before sleeping, and to fast three days of each month."
Reference: Sunan an-Nasa'i 2404
In-book reference: Book 22, Hadith 315
English translation: Vol. 1, Book 22, Hadith 2406