The Prophet ﷺ said, "Fasting three days of every month is like fasting the entire year. The white days are the mornings of the thirteenth, the fourteenth, and the fifteenth."
عن النبي ﷺ قال: صيام ثلاثة أيام من كل شهر صيام الدهر، وأيام البيض صبيحة ثلاث عشرة، وأربع عشرة، وخمس عشرة.
Nasai (2420) narrated it from Makhlad bin Hasan, who said, Ubaydullah narrated to us, from Zayd bin Abu Unaysa, from Abu Ishaq, from Jareer bin Abdullah, and he mentioned it.Hafiz said in Fath al-Bari (4/226), "Its isnad is sahih."Munziri said in Targheeb wal-Tarheeb (1588), "Its isnad is jayyid."