Sunan an-Nasa'i 2413
سنن النسائي#2413SahihIt was narrated that Al-Hurr bin Sayyah said:"I heard Ibn 'Umar say" "The Prophet used to fast three days of each month
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 22:324