مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2720
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 trim his moustache and your father Ibraheem used to trim his moustache before him.
Musnad Ahmad 2738Its isnad is da'eef]