Sahih Muslim 760
Sahih Muslim · Book 6: The Book of Prayer - Travellers · Encouragement to pray qiyam during Ramadan, which is Tarawih
Abu Huraira reported Allah's Apostle (ﷺ) as saying:
حَدَّثَنِي مُحَمَّدُ بْنُ رَافِعٍ ، حَدَّثَنَا شَبَابَةُ ، حَدَّثَنِي وَرْقَاءُ ، عَنْ أَبِي الزِّنَادِ ، عَنِ الأَعْرَجِ ، عَنْ أَبِي هُرَيْرَةَ ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " مَنْ يَقُمْ لَيْلَةَ الْقَدْرِ فَيُوَافِقُهَا - أُرَاهُ قَالَ - إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ " .
He who prayed on the Lailat-ul-Qadr (the Majestic Night) knowing that it is (the same night). I (believe) that he (the Prophet also) said: (He who does) it with faith and seeking reward (from Allah), his sins would be forgiven.