He who observed prayer at night during Ramadan, because of faith and seeking his reward from Allah, his previous sins would be forgiven.
Sahih Muslim Book 6, Hadith 207
← Previous Hadith This hadith is narrated by Ishaq with the same chain uf transmitters except this that the hadith transmitted by Mansur (the above one) is more comprehensive and lengthy.

Sahih Muslim Book 6, Hadith 206 Next Hadith → The Messenger of Allah (ﷺ) used to exhort (his Companions) to pray (at night) during Ramadan without commanding them to observe it as an obligatory act, and say: He who observed the night prayer in Ramadan because of faith and seeking his reward (from Allah), all his previous sins would be forgiven. When Allah's Messenger (ﷺ) died, this was the practice, and it continued thus during Abu Bakr's caliphate and the early part of 'Umar's caliphate.

Sahih Muslim Book 6, Hadith 208