Does the prayer row begin from the right or from the middle?

It is prescribed for the congregants to start the row from behind the Imam, based on the Prophet, peace and blessings be upon him, saying: "Place the Imam in the center and fill the gaps." This supports the idea that the row should begin from that spot. Also, due to his saying: "Let those of you who are mature and discerning stand closest to me," which encourages them to stand behind the Imam.