Sayyiduna Abu Sa'id Khudri رضي الله عنه narrates from the Noble Prophet صلى الله عليه وسلم that he صلى الله عليه وسلم said: "When three people gather together, then one of them should lead them in prayer, and the one most deserving of leading is the one who is the best reciter and most knowledgeable in religion among them." Imam Abu Bakr رحمه الله says that in this hadith, and in the narration of Qatadah from Sayyiduna Abu Qatadah رضي الله عنه from Sayyiduna Abu Sa'id رضي الله عنه, and in the narration of Sayyiduna Aws رضي الله عنه from Sayyiduna Ibn Mas'ud رضي الله عنه, there is evidence that if a slave knows more of the Qur'an than free people, then he is more deserving of leading the prayer, because the Noble Prophet صلى الله عليه وسلم did not exclude the free person from the slave in this narration. (Rather, the ruling is the same for all.)"