Musnad Ahmad 31:665

Surayj narrated to us, Abu Layla narrated to us, my father said—and Surayj named him: Abdullah ibn Maysarah al-Khurasani—from Ghayth al-Bakri, he said: We used to sit with Abu Sa’id al-Khudri in Medina, so I asked him about the seal of the Messenger of Allah, may Allah bless him and grant him peace, which was between his shoulder blades. He said, with his index finger: "Like this, a protruding piece of flesh between his shoulder blades, may Allah bless him and grant him peace."