Jami` at-Tirmidhi Book 49, Hadith 40
"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."
"My maternal aunt took me to the Prophet (ﷺ), and said: 'O Messenger of Allah! Indeed my nephew is in pain.' So he wiped over my head and supplicated for blessings for me. And he performed Wudu and I drank from the water of his Wudu. Then I stood behind his back, and I looked at the seal between his two shoulder blades, and it resembled the egg of a partridge.
"The two shins of the Messenger of Allah (ﷺ) were thin, and he would not laugh except as a smile, and whenever I looked at him I would say: "He put kohl on his eyes,' but he (ﷺ) did not use kohl."