مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2884
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that ibn Abbas (رضي الله عنه) said: The Prophet (ﷺ) used to pray Witr with three Soorahs, with Sabbih isma ribbikal Aala, Qul ya ayyuhal-kafiroon and Qul Huwallahu Ahad.
Musnad Ahmad 2905A saheeh hadeeth]