Sunan an-Nasa'i 5024
It was narrated from Abu Hurairah that :
The Messenger of Allah [SAW] said: "Whoever stands (in he voluntary night prayer of) the Ramadan out of faith and in the hope of reward, his previous sins will be forgiven."
أَخْبَرَنَا قُتَيْبَةُ قَالَ حَدَّثَنَا سُفْيَانُ عَنْ الزُّهْرِيِّ عَنْ أَبِي سَلَمَةَ عَنْ أَبِي هُرَيْرَةَ  أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ مَنْ قَامَ شَهْرَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ