Sunan an-Nasa'i, Book 20, Hadith 5 (In-Book) ⓘ
It was narrated from Abu Hurairah that: The Messenger of Allah said: "Whoever prays Qiyam during Ramadan out of faith and in the hope of reward, he will be forgiven his previous sins."
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
also referenced under: Hadith 1602 (Reference) & Volume 2, Book 20, Hadith 1603 (DarusSalam)