Bulugh al-Maram, Book 5, Hadith 697 (In-Book)

Abu Hurairah narrated that The Messenger of Allah said: “Whoever prays during the night in Ramadan out of sincere faith and seeking its reward from Allah, will have all of his previous sins forgiven." Agreed upon.

عَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ: { مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا, غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ } مُتَّفَقٌ عَلَيْهِ 1‏.‏ ‏1 ‏- صحيح.‏ رواه البخاري ( 2009 )‏، ومسلم ( 759 )‏.‏

sunnah.com

also referenced under: Book 5, Hadith 717 (English)