Musnad Abu Ya'la al-Mawsili 129:115

Abu Musa Muhammad ibn al-Muthanna narrated to us, Abd al-Wahhab narrated to us, Humayd narrated to us, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) was of medium stature, of good build, neither tall nor short. His hair was neither curly nor straight, and he was of a brownish complexion. When he walked, he would lean forward.