The Messenger of Allah ﷺ used to encourage standing in prayer during Ramadan without making it a binding command. He would say, "Whoever stands in prayer during Ramadan out of faith and seeking reward will have his past sins forgiven."The Messenger of Allah ﷺ passed away while the matter remained as such, and it continued in this way during the caliphate of Abu Bakr and the early part of the caliphate of Umar.
حدثنا عبد بن حميد حدثنا عبد الرزاق أخبرنا معمر عن الزهري عن أبي سلمة
كان رسول الله ﷺ يرغب في قيام رمضان من غير أن يأمرهم بعزيمة ويقول من قام رمضان إيمانا واحتسابا غفر له ما تقدم من ذنبه فتوفي رسول الله ﷺ والأمر على ذلك ثم كان الأمر كذلك في خلافة أبي بكر وصدرا من خلافة عمر على ذلك
Muslim also narrated it in the Book of the Prayer of Travelers, Chapter on Encouragement to Perform the Night Prayer in Ramadan, Which Is Tarawih, hadith no. 759, from Abd bin Humayd through the same chain. On this subject there is also a narration from Ayesha [Nasai, no. 2194].