I heard Messenger of Allah ﷺ saying regarding Ramadan, "Whoever prayed at night in it (the month of Ramadan) out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven." (Using translation from Bukhārī 2008)
سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ لِرَمَضَانَ «مَنْ قَامَهُ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ»