Musnad Ahmad 1243:6

Rawh narrated to us, Shu'bah narrated to us, he said: I heard Yahya ibn Husayn say: I heard my grandmother say: I heard the Prophet of Allah, peace and blessings of Allah be upon him, at Arafat delivering a sermon, saying: "May Allah forgive those who shave," three times. They said: "And those who shorten [their hair]?" He said: "And those who shorten," on the fourth time. She said: And I heard him say: "If a slave is appointed over you who leads you by the Book of Allah, then listen to him and obey."