Musnad Ahmad 1014:2

Zayd ibn al-Hubab narrated to us, Husayn ibn Waqid narrated to me, he said: I heard Abu Nahik say: I heard Abu Zayd ‘Amr ibn Akhtab say: "I saw the Seal that was between the shoulders of the Messenger of Allah, may Allah bless him and grant him peace, like a man [a small lump of flesh]." He gestured with his third finger like this, and I wiped it with my hand.