Musnad Ahmad 885:1

Harami ibn 'Umarah narrated to us, he said: 'Uzrah al-Ansari narrated to me, 'Ilba' ibn Ahmar narrated to us, Abu Zayd narrated to us, he said: The Messenger of Allah, may Allah bless him and grant him peace, said to me: "Come close to me." So I came close to him, and he said: "Insert your hand and wipe my back." He said: So I inserted my hand into his shirt and wiped his back, and the Seal of Prophethood fell between my fingers. He said: So he was asked about the Seal of Prophethood, and he said: "Hairs between his shoulder blades."