Sunan an-Nasa'i 2411
سنن النسائي#2411SahihUthman bin Abi Al-As said:"I heard the Messenger of Allah say: 'It is a good fast 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 ZaiIsnaad Sahih
In-book reference: 22:322