مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3508
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 rak'ahs, reciting Sabbih isma Rabbikal-A'la, Qul ya ayyuhal-kafiroon, and Qul Huwallahu Ahad.
Musnad Ahmad 3531Its isnad is saheeh]