Sunan an-Nasa'i 2198
سنن النسائي#2198SahihIt was narrated that Abu Hurairah said:"The Messenger of Allah used to encourage (us) to pray Qiyam during Ramadan, without insisting on that, and he 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: 22:109