Position of a single male follower
Chapter on Imamate and Congregational Prayer
Al-Mughni
Book of Prayer
Primary text
If the follower is a single male, the Sunnah is for him to stand to the Imam's right, whether the person on the right is an adult man or a boy, based on the hadith of Ibn 'Abbas and Anas. Evidence also shows that when Jabar ibn Sakhr came to stand to the Prophet's left, the Prophet moved him and Jabir behind him.
Supporting text
If there are three followers, the Imam stands forward, and the two followers stand behind him. This is the view of 'Umar, 'Ali, Jaber ibn Zayd, Al-Hasan, 'Ata', Al-Shafi'i, and the proponents of opinion. Ibn Mas'ud preferred they all stand in a single row. The preference for standing behind is indicated because the Prophet moved Jabar and Jabar behind him, and in the case of Anas and the orphan, he placed them behind him.