مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2697
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 eight rak'ahs at night, and he would pray Witr with three rak'ahs, then pray the two rak'ahs. When he grew old, he settled with nine (altogether), six and three.
Musnad Ahmad 2714Saheeh]