Abu Dharr narrated that the Messenger of Allah (sallallahou ‘alayhi wa sallam) said : “O Abu Dharr! When you fast three days out of a month, then fast the thirteenth, fourteenth, and fifteenth.”
Narrated by At-Tirmidhî (n°761) and An-Nasâi (n°2424) in a similar hadith.
The Messenger of Allah (sallallahou ‘alayhi wa sallam) said, “Observing Saum (fasting) on three days of every month is equivalent to a full month’s fasting.”
Narrated by Al-Bukhari and Muslim.