The Book of Fasting
22/81. Chapter:
Fasting Three Days of The Month
Sunan an-Nasai
أَخْبَرَنَا مُحَمَّدُ بْنُ عَلِيِّ بْنِ الْحَسَنِ، قَالَ سَمِعْتُ أَبِي قَالَ، أَنْبَأَنَا أَبُو حَمْزَةَ، عَنْ عَاصِمٍ، عَنِ الأَسْوَدِ بْنِ هِلاَلٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ أَمَرَنِي رَسُولُ اللَّهِ صلى الله عليه وسلم بِثَلاَثٍ بِنَوْمٍ عَلَى وِتْرٍ وَالْغُسْلِ يَوْمَ الْجُمُعَةِ وَصَوْمِ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ .
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 2405In-book reference : Book 22, Hadith 316English translation : Vol. 3, Book 22, Hadith 2407