مَنْ قَامَ شَهْرَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ
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."