It wasnarrated that Abu Hurairah said:
"The Messenger of Allah (ﷺ)said: 'Whoever fasts Ramadan and spends its nights in prayer, outoffaith and in hope of reward, his previous sins will be forgiven.'" (RU)
English reference (RU): Vol. 1, Book 5, Hadith 1326 (RU)
Arabic reference (RU): Book 5, Hadith 1387 (RU)