Should the congregant stand abreast of the Imam or slightly behind him in a congregational prayer consisting of two men?

It is obligatory to straighten the rows in prayer, and this is indicated by the hadith of An-Nu'man ibn Bashir: "You must straighten your rows, or Allah will cause discord among your faces." If an Imam and one worshipper stand [together], the worshipper should be abreast of the Imam, neither ahead of him nor behind him. The hadith of Ibn Abbas clarifies that the Prophet, peace and blessings be upon him, placed him beside him. The Imam and the worshipper are considered one row, which requires straightening so that no one is ahead of anyone else.