Sahih Al Bukhari 1: Chapter 2, Hadith 35

(35) To establish the (Nawafil - voluntary) prayers on the night of Qadr is a part of faith

حَدَّثَنَا أَبُو الْيَمَانِ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ أَخْبَرَنَا شُعَيْبٌ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا أَبُو الزِّنَادِ، ‏‏‏‏‏‏عَنِ الْأَعْرَجِ، ‏‏‏‏‏‏عَنْ أَبِي هُرَيْرَةَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ:‏‏‏‏ "مَنْ يَقُمْ لَيْلَةَ الْقَدْرِ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ".

SAHIH HADITH:

Narrated Abu Huraira: Allah's Apostle said, Whoever establishes the prayers on the night of Qadr out of sincere faith and hoping to attain Allah's rewards (not to show off) then all his past sins will be forgiven.

REFERENCE:

Sahih Al Bukhari 1: Chapter 2, Hadith 35