كتاب الصيام
5  
Fasting
Bulugh al-Maram 697
Abu Hurairah narrated that The Messenger of Allah (ﷺ) 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 .‏
Reference : Bulugh al-Maram 697
In-book reference : Book 5, Hadith 697
English translation : Book 5, Hadith 717
Hadith Text
Reference