Sunan an-Nasa'i 2252
سنن النسائي#2252Sahihأَخْبَرَنَا أَحْمَدُ بْنُ حَرْبٍ، قَالَ حَدَّثَنَا قَاسِمٌ، عَنْ سُفْيَانَ، عَنْ سُهَيْلِ بْنِ أَبِي صَالِحٍ، عَنِ النُّعْمَانِ بْنِ أَبِي عَيَّاشٍ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " مَنْ صَامَ يَوْمًا فِي سَبِيلِ اللَّهِ بَاعَدَ اللَّهُ بِذَلِكَ الْيَوْمِ حَرَّ جَهَنَّمَ عَنْ وَجْهِهِ سَبْعِينَ خَرِيفًا " .
It was narrated from Abu Saeed Al-Khudri that the Prophet said:"Whoever fasts a day in the cause of Allah, Allah will separate his face from the heat of Hell (a distance of) seventy autumns
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 22:163