Language:
كتاب الإيمان
2
Belief
(27)
Chapter: It is a part of faith to establish the (Nawafil — voluntary) prayers during the nights of Ramadan
(27)
باب تَطَوُّعُ قِيَامِ رَمَضَانَ مِنَ الإِيمَانِ

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."

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 37
In-book reference : Book 2, Hadith 30
USC-MSA web (English) reference : Vol. 1, Book 2, Hadith 37
  (deprecated numbering scheme)