Musnad Ahmad 30:1657

Mansur ibn Salamah Abu Salamah al-Khuza'i narrated to us, he said: Sulayman ibn Bilal informed us, from al-'Ala' ibn 'Abd al-Rahman, from his father, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, said: "Trim the mustaches, and let the beards grow."