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