Sunan an-Nasa'i, Book 22, Hadith 116 (In-Book) ⓘ
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 عَلِيُّ بْنُ الْمُنْذِرِ، قَالَ حَدَّثَنَا ابْنُ فُضَيْلٍ، قَالَ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، رضى الله عنه قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ صَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
also referenced under: Hadith 2205 (Reference) & Volume 3, Book 22, Hadith 2207 (DarusSalam)