Musnad Ahmad 1240:3

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Yazid ibn Abi Habib al-Masri narrated to me, from Abd al-Rahman ibn Uqbah, the freed slave of Ma'mar ibn Abdullah ibn Nafi' ibn Nadlah al-Adawi, from Ma'mar ibn Abdullah, who said: I used to saddle the mount for the Messenger of Allah (peace and blessings of Allah be upon him) during the Farewell Pilgrimage. He said: He said to me one night: "O Ma'mar, I have found a disturbance in my saddle straps tonight." I said: "By the One who sent you with the truth, I have tightened them just as I used to tighten them, but the one who has envied my position with you has loosened them, so that you might replace me with someone else." He said: He replied: "As for that, I will not do it." He said: When the Messenger of Allah (peace and blessings of Allah be upon him) slaughtered his sacrificial animal at Mina, he ordered me to shave his head. I took the razor and stood by his head. The Messenger of Allah (peace and blessings of Allah be upon him) looked at my face and said to me: "O Ma'mar, the Messenger of Allah has placed his earlobe in your power, while the razor is in your hand?" I said: "By Allah, O Messenger of Allah, that is indeed from the favor of Allah upon me and His grace." He said: He replied: "Yes, if it is acknowledged for you." He said: Then I shaved the Messenger of Allah (peace and blessings of Allah be upon him).