Abu 'Uthman al-Nahdi عبد الرحمن بن مل - أبو عثمان النهدي · The Book of Fasting

"The Messenger of Allah said to a man: 'Come to the blessed breakfast, - meaning Sahur." '

"The Messenger of Allah said to me: 'Fast one day of the month and you will have the reward of what is left.' I said: 'I am able to do more than that.' He said: 'Fast two days and you will have the reward of what is left.' I said: 'I am able to do more than that.' He said: 'Fast three days and you will have the reward of what is left.' I said: 'I am able to do…

"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."

"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).