The seal of the Prophethood is a physical mark that every Prophet had. The Prophet Muhammad had this seal on his back. It is described as the outgrowth of hair in an area where hair does not grow and it was of a different color. It was between his shoulder blades shaped like a pigeon’s egg. Ali ibn Talib, referring to this seal and the Prophet Muhammad , says,
He had this seal and he was the seal of the prophets.
Salman al-Farsi, a famous companion of the Prophet converted to Islam because his christian teacher told him that the sign of the prophet is the seal between his shoulder blades.