Sahih al-Bukhari 37
Sahih al-Bukhari · Book 2: Belief · It is a part of faith to establish the (Nawafil — voluntary) prayers during the nights of Ramadan
Narrated Abu Huraira:
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 " .
Allah's Messenger (ﷺ) 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."