كتاب الفضائل
9
The Book of Virtues
Abu Hurairah (May Allah be pleased with him) reported:
The Prophet (ﷺ) said, "Whosoever performs Qiyam during Lailat-ul-Qadr (Night of Decree), with Faith and being hopeful of Allah's reward, will have his former sins forgiven."

[Al-Bukhari and Muslim].

وعن أبي هريرة رضي الله عنه، عن النبي صلى الله عليه وسلم قال‏:‏ ‏ "‏من قام ليلة القدر إيمانا واحتسابًا غفر له ما تقدم من ذنبه‏"‏‏.‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏ ‏.‏
Sunnah.com reference : Book 9, Hadith 199
Arabic/English book reference : Book 9, Hadith 1189