Sunan an-Nasa'i 2202
سنن النسائي#2202SahihIt was narrated from Abu Hurairah that the Prophet said:"Whoever fasts during Ramadan" and according to the Hadith of Qutaibah, the Prophet said: "Whoever spends the nights of Ramadan in prayer (Qiyam) out of faith and in the hope of reward, he will be forgiven his previous sins, and whoever spends the night of Lailat Al-Qadr in prayer out of faith and in the hope of reward, he will be forgiven his previous sins
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiHasan
In-book reference: 22:113