Musnad Abu Ya'la al-Mawsili 183:2

'Amr ibn al-Dahhak narrated to us, my father narrated to us, 'Uzrah ibn Thabit 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: "Draw near to me and wipe my back." He said: So I uncovered his back and wiped his back. He said: And I placed the Seal between my fingers and pressed it. He said: It was said, "And what is the Seal?" He said: "Hair gathered together on his shoulder."