مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2709
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) used to pray Witr with three (rak'ahs)... and he narrated a similar report.
Musnad Ahmad 2726Its isnad is saheeh]