Sunan an-Nasa'i 2191
سنن النسائي#2191SahihIt was narrated from Saeed bin Al-Musayyab 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 Lighairihi
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 22:102