Sunan an Nasai
2192
Hasan Sahih+2

Urwah bin Az-Zubair narrated that Aishah told him that:the Messenger of Allah used to encourage the people to pray Qiyam in Ramadan, without insisting on that. He said: "Whoever spends the nights of Ramadan in prayer out of faith and in the hope of reward, he will be forgiven his previous sins

Hasan SahihAbu Ghuddah
Hasan SahihAl-Albani