Muhammad bin Maymun, Abu Hamza محمد بن ميمون المروزي أبو حمزة السكري · The Book of Fasting

"The Messenger of Allah used to be keen to fast on Mondays and Thursday."

"The Messenger of Allah said to me: 'Read the Qur'an in a month.' I said: 'I am able to do more than that. ' And I kept asking him until he said: 'In five days.' And he said: 'Fast three days a month.' I said: 'I am able to do more than that.' And 'Observe thhe most beloved of fasting to Allah, the Mighty and Sublime, the fast one day and not the next." (Suahih).