Sunan Abi Dawud 2450
سنن أبي داود#2450HasanNarrated Abdullah ibn Mas'ud: The Messenger of Allah (ﷺ) used to fast three days every month
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniHasan
Muhammad Muhyi Al-Din Abdul HamidHasan
Shuaib Al ArnautHasan
Zubair Ali ZaiIsnaad Hasan
In-book reference: 14:138