Hadith
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 ".
Narrated Abu Huraira: 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
Sahih
Sahih al-Bukhari 37 ·Book 2, Hadith 30·Also available on sunnah.com·