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

"The Messenger of Allah said: 'Whoever fasts three days in every month, then that is (similar to) fasting every day.' Then Allah Mighty and Sublime is He, attested to that in His Book, by revealing: Whoever brings a good deed, shall have ten times the like thereof. So a day is like ten."