Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2205 (DarusSalam) ⓘ
It was narrated from Abu Hurairah that the Prophet said: "Whoever fasts in Ramadan out of faith and in the hope of reward, he will be forgiven his previous sins." '
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " مَنْ صَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
also referenced under: Hadith 2203 (Reference) & Book 22, Hadith 114 (In-Book)