It was the practice of the Prophet (peace and blessings be upon him):
"The Seal of Prophethood was on his (peace and blessings be upon him) back in the form of a raised piece of flesh."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4807
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ت في الشمائل] عن أبي سعيد. الصحيحة ٢٠٩٣: حم.