It was the practice of the Prophet (peace and blessings be upon him):
"His (peace and blessings be upon him) Seal of Prophethood was like a red gland, the size of a pigeon's egg."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4808
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ت] عن جابر بن سمرة. الصحيحة ٢٠٩٣: م، ابن سعد.