KHATIMU ‘N-NABU WAH خاتم النبوة
“The seal of prophecy.” A term used for the large mole or fleshy protuberance on Muhammad’s back, which is said to have been a divine sign of his prophetic office.
‘Abdu ‘llah ibn Sarjis describes it as being as large as his closed fist, with moles round about it. Abu Ramsah wanted to remove it, but Muhammad refused saying, “The Physician thereof is He who place it there.”

Based on Hughes, Dictionary of Islam