Sunan an-Nasa'i 2251
سنن النسائي#2251SahihIt was narrated that Abu Saeed Al-khudi said:"The Messenger of Allah: 'There is no worshipper who fasts a day in the cause of Allah, but Allah, the most high, will separate (a distance of) seventy autumns between his face and the fire 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 ZaiHasan
In-book reference: 22:162