"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, and whoever spends the night of Lailat Al-Qadr in prayer out of faith and in the hope of reward, he will be forgiven his previous sins."'
Reference: Sunan an-Nasa'i 2207 In-book : Book 22, Hadith 118 English translation : Vol. 3, Book 22, Hadith 2209
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