The Messenger of Allah ﷺ used to encourage standing in prayer during Ramadan without making it a strict obligation. He said, "Whoever stands in prayer during Ramadan out of faith and seeking reward will have his past sins forgiven."
أخبرنا نوح بن حبيب قال حدثنا عبد الرزاق قال أنبأنا معمر عن الزهري عن أبي سلمة
كان رسول الله ﷺ يرغب في قيام رمضان من غير أن يأمرهم بعزيمة قال من قام رمضان إيمانا واحتسابا غفر له ما تقدم من ذنبه
[Sahih], previously mentioned, no. 2196. Muslim also narrated it in the chapter on prayer for travelers, the section on encouraging the night prayers of Ramadan, which is tarawih, no. 174/759, from the hadith of Abdul-Razzaq, with the same wording. It is also found in Al-Kubra, no. 2508.