Sunan an-Nasa'i 2248
سنن النسائي#2248SahihIt was narrated from Abu Saeed that he heard the Messenger of Allah say:"There is no worshipper who fasts a day in the cause of Allah, the mighty and sublime, but Allah the mighty and sublime, will separate his face from the fire by (a distance of) seventy autumns in return for that day
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 - Agreed Upon
In-book reference: 22:159