Musnad Abu Ya'la al-Mawsili 111:103

Zuhayr ibn Harb narrated to us, Yahya ibn Abi Bakr narrated to us, al-Hasan ibn Salih narrated to us, from Simak, from Ikrimah, from Ibn Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to trim his mustache, and Ibrahim used to trim his mustache.