Messenger of Allah ﷺ 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."  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ يَقُمْ لَيْلَةَ الْقَدْرِ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ