Musnad Ahmad 30:1551

Yahya ibn Ishaq narrated to us, Abu 'Awanah narrated to us, from 'Umar ibn Abi Salamah, from his father, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Let the beards grow, and trim the mustaches, and change your gray hair, and do not resemble the Jews and the Christians."