Sunan an-Nasa'i 2414
سنن النسائي#2414SahihIt was narrated from Ibn 'Umar that:the Messenger of Allah used to fast three days of each month: Monday at the beginning of the month, the following Thursday, then the Thursday after that
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih Lighairihi
Al-AlbaniSahih
Zubair Ali ZaiHasan
In-book reference: 22:325