Sunan an-Nasa'i 2199
سنن النسائي#2199SahihIt was narrated from Abu Hurairah that the Messenger of Allah 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
In-book reference: 22:110