Isa bin Msawr al-Jwhry عيسى بن مساور الجوهري · The Book of Fasting

"The Messenger of Allah commanded me to pray two Rakahs of Duha, and not to sleep until I had prayed witr, and to fast three days each month."

"The Messenger of Allah used to command us to fast the three days with the shining bright nights (Al-Ayam Al-Bid), the thirteenth, fourteenth and fifteenth."