Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1417
Abu Sa'id al-Khudri reported Allah's Messenger (may peace be upon him) as saying: When there are three persons, one of them should lead them. The one among them most worthy to act as Imam is one who is best versed in the Qur'an.