The Messenger of Allah ﷺ said, "Whoever stands in prayer during Ramadan out of faith and seeking reward will have his past sins forgiven."
أخبرنا قتيبة عن مالك عن ابن شهاب عن حميد بن عبد الرحمن
أن رسول الله ﷺ قال من قام رمضان إيمانا واحتسابا غفر له ما تقدم من ذنبه
Bukhari narrated it in Salat al-Tarawih, chapter on the merit of one who stands in prayer during Ramadan, hadith no. 2009, and hadith no. 37, and Muslim narrated it in Salat al-Musafirin, chapter on encouragement to stand in prayer during Ramadan, which is tarawih, hadith no. 759, from the hadith of Malik through this chain. It is also in Al-Kubra, hadith no. 1295, and in Muwatta, narration of Abu Musab Zuhri, 1/109, hadith no. 278.