Sunan an-Nasa'i 2197
سنن النسائي#2197SahihIt was narrated from Ibn Shahab that Abu Salamah told him that Abu Hurairah said:"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
Zubair Ali ZaiHasan
In-book reference: 22:108