Musannaf Ibn Abi Shayba 3:6485

Abu Bakr narrated to us, he said: Hushaym narrated to us, from al-‘Awwam, from ‘Abd al-Malik al-Taymi, from Ibrahim, who said: The beginning of the row is directly behind the Imam. If there is no one with the Imam except one person, he should place him behind him, between himself and the bowing. If someone comes, he should pray with him. If no one comes until he bows, he should join the Imam and stand to his right. If he comes and the row is full, he should stand directly behind the Imam. If someone comes, he should pray with him. If no one comes, he should enter into the row, and then [do] likewise and likewise.