Sunan an-Nasa'i, Hadith 5025 ⓘ
It was narrated from Abu Hurairah that : The Prophet [SAW] said: "Whoever stands (in the voluntary night prayer of) Ramadan out of faith and in hope of reward, his previous sins will be forgiven."
أَخْبَرَنَا قُتَيْبَةُ عَنْ مَالِكٍ عَنْ ابْنِ شِهَابٍ ح وَالْحَارِثُ بْنُ مِسْكِينٍ قِرَاءَةً عَلَيْهِ وَأَنَا أَسْمَعُ عَنْ ابْنِ الْقَاسِمِ قَالَ حَدَّثَنِي مَالِكٌ عَنْ ابْنِ شِهَابٍ عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ عَنْ أَبِي هُرَيْرَةَ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ
also referenced under: Book 47, Hadith 41 (In-Book) & Volume 6, Book 47, Hadith 5028 (DarusSalam)