Musnad Ahmad 31:324

Abu 'Amir narrated to us, Hisham narrated to us, from Qatadah, from Abu Nadrah, from Abu Sa'id al-Khudri, that the Prophet (peace and blessings of Allah be upon him) said: "If three are gathered, let one of them lead them in prayer, and the most entitled of them to the leadership is the one who is most proficient in reciting the Quran."