سنن أبي داود

Narrated Abu Hurairah: The Messenger of Allah (ﷺ) used to commend prayer at night during Ramadan, but did not command it as duty. He would say: If anyone prays during the night in Ramadan because of faith and seeking his reward from Allah, his previous sins will be forgiven for him. When the Messenger of Allah (ﷺ) died, this was the practice, and it continued thus during Abu Bakr's caliphate and early part of 'Umar's.

Abu Dawud said: This tradition has been transmitted by 'Uqail, Yunus, and Abu Uwais in like manner. The version of 'Uqail goes: He who fasts during Ramadan and prays during the night.