Musnad Ahmad 1014:9

Abu 'Asim narrated to us, 'Uzrah narrated to us, 'Ilba' ibn Ahmar narrated to us, Abu Zayd narrated to us, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "O Abu Zayd, come near to me and wipe my back." And he uncovered his back, so I wiped his back, and I placed the Seal [of Prophethood] between my fingers. He said: So I pressed it. He said: It was said: "And what is the Seal?" He said: "Hair gathered together on his shoulder."