مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ
It was narrated from Abu Hurairah that: The Prophet [SAW] said: "Whoever stands (in the voluntary night prayer of) Ramadan out of faith and in the hope of reward, his previous sins will be forgiven."