Messenger of Allah ﷺ said: "Whoever establishes prayers during the nights of Ramadan faithfully out of sincere faith and hoping to attain Allah's rewards (not for showing off), all his past sins will be forgiven." (Using translation from Bukhārī 37)
قَالَ رَسُولُ اللهِ ﷺ «مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ»