PROPHET MUHAMMAD (ﷺ) said:
Fasting three days of EACH MONTH is fasting for a lifetime, and the shining days of Al-Bid, the thirteenth, fourteenth and fifteenth.

Narrated: Abu Dhar
The Messenger of Allah (ﷺ) commanded us to fast the three days of Al-Bid, the thirteenth, fourteenth and fifteenth.

PROPHET MUHAMMAD (ﷺ) said:
If you want to fast at any time during the month then fast on the thirteenth, fourteenth and fifteenth.

Narrated: Abu Dhar
"The Messenger of Allah (ﷺ) said:
'Whoever fasts three days in every month, then that is (similar to) fasting every day (life time).' Then Allah Mighty and Sublime is He, attested to that in His Book, by revealing: (Quran 6:160) Whoever brings a good deed, shall have ten times the like thereof. So a day is like ten."