Riyad as-Salihin 1188
Abu Hurairah (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) used to urge (the people) to perform (optional Tarawih) prayer at night during the month of Ramadan. He did not order them or make it obligatory on them. He (ﷺ) said, "Whosoever performs (optional Tarawih) prayers at night during the month of Ramadan, with Faith and in the hope of receiving Allah's reward, will have his past sins forgiven." [Muslim] .
وعنه، رضي الله عنه قال‏:‏ كان رسول الله صلى الله عليه وسلم، يرغب في قيام رمضان من غير أن يأمرهم فيه بعزيمة، فيقول‏:‏ ‏
"‏من قام رمضان إيمانًا واحتسابًا غفر له ما تقدم من ذنبه‏"‏ ‏.‏ ‏(‏‏(‏رواه مسلم‏)‏‏)‏‏.‏