Musnad Ahmad 907:128

'Abdullah narrated to us, Yahya bin 'Abdullah, the freed slave of Banu Hashim, narrated to me in the year two hundred and twenty-nine, Shu'bah narrated to us, from Simak, from Jabir bin Samurah, who said: "I saw the seal between the shoulders of the Prophet, may Allah bless him and grant him peace, as if it were a pigeon's egg."