The Messenger of Allah ﷺ used to encourage standing in prayer during Ramadan without making it a strict obligation, and he would say, "Whoever stands in prayer during Ramadan out of faith and seeking reward, his previous sins will be forgiven."
حدثنا عثمان بن عمر حدثنا مالك عن الزهري عن أبي سلمة
أن رسول الله ﷺ كان يأمر بقيام رمضان من غير أن يأمر فيه بعزيمة وكان يقول من قام رمضان إيمانا واحتسابا غفر له ما تقدم من ذنبه
Its isnad is sahih. It has occurred previously at 7774, without the statement of Abu Hurairah, and it has also occurred previously at 10485.