Depends on how you define "credible." There are certainly Muslim sources which describe his appearance. Non-Muslims sources? None that I know of. That's to be expected though as most of Arabia had become Muslim by the time of his passing.

So, with that said, these are from Muslim sources:

From his servant Anas ibn Malik: 'He was of average height, not too tall and not too short, with a pinkish colour, not very white and not dark, and his hair was neither very curly nor very straight..... When he died, there were no more than twenty white hairs on his head and in his beard." (al-Bukhaari, 3283)

"The Messenger of Allaah was broad shouldered and had thick hair coming down to his shoulders and earlobes. He was wearing red garments." (Muslim).

From his son in law Ali ibn Abi Talib: "He was neither tall nor short, and had large hands and feet. He had a large head and was big-boned, and the thin line of hair (starting from his chest and extending to the navel) was long. When he walked, he would lean forward, as if he was walking downhill. I have never seen anyone like him, before or since." (Tirmidhi, 3570)