the Messenger of Allah said: "O Abu Dharr! When you fast three days out of a month, then fast the thirteenth, fourteenth, and fifteenth."
Jami` at-Tirmidhi Book 8, Hadith 80
← Previous Hadith "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."

Jami` at-Tirmidhi Book 8, Hadith 79 Next Hadith → "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."

Jami` at-Tirmidhi Book 8, Hadith 81