Sunan an-Nasa'i, Hadith 2204 ⓘ
It was narrated that Abu Hurairah said: "The Messenger of Allah said: 'Whoever fasts Ramadan out of faith and in the hope of reward, he will be forgiven his previous sins."'
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ صَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
also referenced under: Book 22, Hadith 115 (In-Book) & Volume 3, Book 22, Hadith 2206 (DarusSalam)