Musnad Abu Ya'la al-Mawsili 154:108

Zuhayr narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, who said: I heard Abd al-Rahman ibn Alqamah say: I heard Ibn Umar say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let the beards grow long, and trim the mustaches."