From Sahih Muslim (Hadith 1781):
Abu Huraira reported Allah's Messenger (ﷺ) as saying:He who observed the fasts of Ramadan with faith and seeking reward (from Allah), all his previous sins would be forgiven, and he who observed prayer on Lailat-ul- Qadr with faith and seeking reward (from Allah), all his previous sins would be forgiven
Reference: Muslim 1781