or should the follower stand in line with him?

The answer is that he stands level with the follower, for when there is an imam and one congregant they together form a single row. It is impossible for the follower to be behind the imam by himself; they are one row, and a single row must be straightened. This contradicts the view of some scholars who say the imam should step slightly forward, a position for which no evidence exists. On the contrary, the proof supports the opposite: when the worshippers are two, the imam and the follower should align shoulder-to-shoulder.

In another wording of the ḥadīth it says: “The Prophet ﷺ would straighten our rows as though he were aligning arrow-shafts.” Arrow-shafts were carefully trimmed so that no feather projected beyond another—level like the teeth of a well-made comb. In the same way the Prophet ﷺ would level the rows. Once he perceived that we had understood and grasped the necessity of keeping the line straight, he came out one day and noticed a man whose chest jutted slightly ahead of the row. He said: “Servants of Allah, straighten your rows, or Allah will cause your faces to diverge.” This shows the reason the Prophet ﷺ uttered the threat: he had merely seen one man’s chest protruding a little. It demonstrates that part of his practice was to inspect the rows and that he warned anyone who stepped forward with the solemn admonition: “Straighten your rows, or Allah will cause your faces to diverge.”

Therefore we must explain this matter to the imams of the mosques and to the congregants alike, so that they pay attention and give proper care to aligning the rows, without becoming lax in this regard. Allah is the One who grants success.