"The Messenger of Allah said: 'Whoever spends the nights of Ramadan in prayer (Qiyam) out of faith and in the hope of reward, he will be forgiven his previous sins." ' (Using translation from Nasāʾī 2197)
وَقَالَ رَسُولُ اللهِ ﷺ مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا فَإِنَّهُ يُغْفَرُ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ