Musnad Ahmad 962:146

Zayd ibn Yahya narrated to us, Abdullah ibn al-Ala ibn Zabr narrated to us, al-Qasim narrated to me, he said: I heard Abu Umamah saying: The Messenger of Allah (peace and blessings of Allah be upon him) went out to some elders of the Ansar whose beards were white, and he said: "O assembly of the Ansar, dye [your beards] red and yellow, and differ from the People of the Scripture." He said: So we said: "O Messenger of Allah, the People of the Scripture wear trousers and do not wear the izar (waist-wrapper)." The Messenger of Allah (peace and blessings of Allah be upon him) said: "Wear trousers and wear the izar, and differ from the People of the Scripture." He said: So we said: "O Messenger of Allah, the People of the Scripture wear khuff (leather socks) and do not wear sandals." The Prophet (peace and blessings of Allah be upon him) said: "Wear khuff and wear sandals, and differ from the People of the Scripture." He said: So we said: "O Messenger of Allah, the People of the Scripture trim their beards and let their mustaches grow." The Prophet (peace and blessings of Allah be upon him) said: "Trim your mustaches and let your beards grow, and differ from the People of the Scripture."