Jami at-Tirmidhi 760
Jami at-Tirmidhi · Book 8: The Book on Fasting · What Has Been Related About Fasting Three (Days) Of Every Month
Abu Hurairah narrated:
حَدَّثَنَا قُتَيْبَةُ ، حَدَّثَنَا أَبُو عَوَانَةَ ، عَنْ سِمَاكِ بْنِ حَرْبٍ ، عَنْ أَبِي الرَّبِيعِ ، عَنْ أَبِي هُرَيْرَةَ ، قَالَ عَهِدَ إِلَىَّ النَّبِيُّ صلى الله عليه وسلم ثَلاَثَةً أَنْ لاَ أَنَامَ إِلاَّ عَلَى وِتْرٍ وَصَوْمَ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَأَنْ أُصَلِّيَ الضُّحَى .
"The Messenger of Allah took a covenant from me for three: To not sleep except after performing Witr, to fast three days of every month, and to perform the Duha prayer."
Grade: Hasan (Darussalam)