Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2249 (DarusSalam) ⓘ
It was narrated from Abu Sa`eed that the Prophet said: "Whoever fasts one day in the cause of Allah, the Mighty and Sublime, Allah will separate his face from the fire by (a distance of) seventy years."
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا مُحَمَّدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ سُهَيْلٍ، عَنْ صَفْوَانَ، عَنْ أَبِي سَعِيدٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " مَنْ صَامَ يَوْمًا فِي سَبِيلِ اللَّهِ عَزَّ وَجَلَّ بَاعَدَ اللَّهُ وَجْهَهُ مِنْ جَهَنَّمَ سَبْعِينَ عَامًا " .
also referenced under: Hadith 2247 (Reference) & Book 22, Hadith 158 (In-Book)