Sunan Abi Dawud Book 2, Hadith 199
When the time of prayer comes, call the Adhan, then call the iqamah, then the one who is oldest of you should act as your imam.
The version narrated by Maslamah goes : He said: On that day we were almost equal in knowledge.
The version narrated by Isma’il says: Khalid said: I said to Abu Qilabah: where is the Qur’an(i.e. why did the Prophet(ﷺ) not say: The one who knows the Qur’an most should act as imam)? He replied: Both of them were equal in the knowledge of the Qur’an.
when the first emigrants came (to Madina), they stayed at al-‘Asbah (a place near Madina) before the advent of the Messenger of Allah (ﷺ). Salim, the client of Abu Hudhaifah, acted as their imam, as he knew the Qur’an better than all of them, al-Haitham(the narrator) added: and ‘Umar b. al-Khattab and Abu Salamah b. ‘Abd al-Asad were among them.
Let the best among you call the adhan for you, and the Qur'an-readers act as your imams.