Sahih al-Bukhari, Volume 1, Book 2, Hadith 36 (USC-MSA) ⓘ
Narrated Abu Huraira: Allah's Messenger said: "Whoever establishes prayers during the nights of Ramadan faithfully out of sincere faith and hoping to attain Allah's rewards (not for showing off), all his past sins will be forgiven."
حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي مَالِكٌ، 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: Book 2, Hadith 30 (In-Book) & Hadith 37 (DarusSalam)