Sunan An-Nasai, Chapter: The Book of Leading the Prayer (Al-Imamah)

"Riders used to pass by us and we would learn the Quran from them. My father came to the Prophet (saw) and he said: 'Let the one of you who knows most Quran lead you in prayer.' My father came and said that the Messenger of Allah (saw) had said: 'Let the one of you who knows most Quran lead you in prayer.' They looked and found that I was the one who knew most Quran, so I used to lead them in prayer when I was eight years old." (Sahih)