Musnad Abu Ya'la al-Mawsili 248:34

Ishaq narrated to us, al-Nadr ibn Shumayl narrated to us, Shu'bah narrated to us, from Simak ibn Harb, from Jabir ibn Samurah, who said: I saw the Seal of Prophethood between the shoulders of the Prophet, may Allah bless him and grant him peace, as if it were a pigeon's egg.