وَعَنْ أَبِي سَعِيدٍ اَلْخُدْرِيِّ رضى الله عنه
قَالَ: قَالَ رَسُولُ اَللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ -«مَا مِنْ عَبْدٍ يَصُومُ يَوْمًا فِي سَبِيلِ اَللَّهِ إِلَّا بَاعَدَ اَللَّهُ بِذَلِكَ اَلْيَوْمِ عَنْ وَجْهِهِ اَلنَّارَ سَبْعِينَ خَرِيفًا»
“No servant (of Allah) fasts on a day (merely) for the sake of Allah except that Allah pushes the Hellfire seventy years further away from his face, due to fasting on this day.” Agreed upon and the wording is from Muslim.
Bulugh al-Maram 790, Book 5, Fasting, Hadith 33. 1 min read