The Book of Mosques and Places of Prayer
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ قَتَادَةَ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا كَانُوا ثَلاَثَةً فَلْيَؤُمَّهُمْ أَحَدُهُمْ وَأَحَقُّهُمْ بِالإِمَامَةِ أَقْرَؤُهُمْ " .Abu Sa'id al-Khudri reported Allah's Messenger (ﷺ) 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
From Sahih Muslim, whose reports the scholars accept as authentic.