Sayyidina Abu Dharr (RA) narrated that Allah’s Messenger (SAW) said, “if anyone keeps fast for three days every month then that is perpetual fasting. Indeed, Allah, the Blessed and Exalted, revealed a confirmation of that in His Book {Whoever comes with a good deed, receives ten times as much. (6 : 160)}. So, each is like ten days.
[Ahmed21359, Nisai 2405, Ibn e Majah 1708]