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