Sunan an-Nasa'i, Hadith 2245 ⓘ
It was narrated that Abu Saeed Al-Khudri said: "The Messenger of Allah said: 'Whoever fasts one day in the cause of Allah, Allah will separate between his face and the fire by (a distance of) seventy autumns in return for that day."'
أَخْبَرَنَا دَاوُدُ بْنُ سُلَيْمَانَ بْنِ حَفْصٍ، قَالَ حَدَّثَنَا أَبُو مُعَاوِيَةَ الضَّرِيرُ، عَنْ سُهَيْلٍ، عَنِ الْمَقْبُرِيِّ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ صَامَ يَوْمًا فِي سَبِيلِ اللَّهِ بَاعَدَ اللَّهُ بَيْنَهُ وَبَيْنَ النَّارِ بِذَلِكَ الْيَوْمِ سَبْعِينَ خَرِيفًا " .
also referenced under: Book 22, Hadith 156 (In-Book) & Volume 3, Book 22, Hadith 2247 (DarusSalam)