Sunan an-Nasa'i 2419
سنن النسائي#2419SahihIt was narrated that Umm Salamah said:"The Messenger of Allah used to enjoin fasting three days: The first Thursday, and Monday and Monday
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahShadh
Al-AlbaniShadh
Zubair Ali ZaiSahih
In-book reference: 22:330