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." (FR)
English reference (FR): Vol. 1, Book 46, Hadith 3644 (FR)
Arabic reference (FR): Book 49, Hadith 4005 (FR)