Musannaf Ibn Abi Shaybah

Ibrahim says that the row should begin from the side of the imam. If there is only one person with the imam, he should stand behind him at such a distance that he can perform bowing (ruku). If another person comes, the imam will lead him in prayer as well. If no one comes until the imam goes into ruku, the person standing behind should join the imam and stand on his right side. If a person comes to pray and the row is complete, he should stand in the direction of the imam; if another comes, he should pray with him. If no other person comes, he should join the row. Then this sequence will continue in the same manner.

Hasan says that when a person arrives and the row is complete, he should stand in line with the Imam.