Sunan an Nasai
2191
Sahih Lighairihi+2

It 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

Sahih LighairihiAbu Ghuddah
SahihAl-Albani