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 محمد بن عبد الله بن عبد الحكم عن شعيب عن الليث قال أنبأنا خالد عن ابن أبي هلال عن ابن شهاب
عن رسول الله ﷺ قال من قام رمضان إيمانا واحتسابا غفر له ما تقدم من ذنبه
[Sahih]. It is in Kubra, hadith no. 2501, and the hadith has very many corroborating narrations (see the following hadith). Shuayb is the son of Layth bin Saad, Khalid is the son of Yazid, and Ibn Abu Hilal is Saeed.