Narrated Jabir bin Samurah:
"The seal of the Messenger of Allah (ﷺ)" - meaning the one which was between his two shoulder blades - "was fleshy and red, resembling the egg of a pigeon." (ID)
English reference (ID): Vol. 1, Book 46, Hadith 3644 (ID)
Arabic reference (ID): Book 49, Hadith 4005 (ID)