—whether by seeing the imam himself or any part of the row, or by hearing him, or through a relayer.

If both worshipper and imam are inside the same mosque, following is valid no matter how great the distance between them, even if walls intervene.
When they stand in an open area, however, the distance between them may not exceed roughly three-hundred cubits; some jurists require that exact figure.
Where two individuals—or two rows—are connected, the distance is reckoned from the rearmost follower to the foremost imam.
It makes no difference whether the open ground is privately owned, endowed, or partly each.
A thoroughfare that people use, or a river that would oblige one to swim across, does not invalidate the connection, according to the sound position.

If the two are in separate structures, such as a courtyard and a portico or two adjoining houses, the jurists have two views.
The sounder is that, when the follower’s building lies to the imam’s right or left, at least one row must stretch from one building into the other; a gap wide enough for a single person to stand does no harm according to the more reliable opinion.
If the follower is behind the imam’s building, the correct view is that following is valid provided that no more than three cubits separate the two rows.
According to the second view, mere closeness—exactly as in the open—suffices, so long as no barrier intervenes, or only an open doorway.
If what lies between them prevents passage but not sight, there are two opinions; if it is a solid wall, the prayer is invalid by agreement of both views.

I say: the second view is sounder; Allah knows best.
Once his own following is valid in a different building, those standing behind him may also follow, even if a wall stands between them and the imam.
If the follower is on an upper level and the imam below, or the reverse, it is required that some part of his body line up with some part of the imam’s body.
Should the follower pray in open land while the imam is inside a mosque, then—provided nothing intervenes—the only requirement is proximity, measured from the rear wall of the mosque; some measure from the last row.
If, however, a wall or a closed door stands between them, the connection is invalid, and the same applies, according to the stronger opinion, to a shut door that merely swings to and a latticed window.

I say: it is disliked for the follower to stand higher than his imam, or the imam higher than his followers, unless need dictates, in which case it is recommended.
One should not rise for prayer until the muʾadhin has finished the iqāmah, nor begin a supererogatory prayer once the iqāmah has commenced—unless he fears missing the congregation.
Allah knows best.

Section

For a follower’s prayer to be valid, he must intend—together with the opening takbīr—to follow the imam or to join the congregation; Friday prayer is no different in this regard, according to the stronger opinion.
If he omits this intention yet imitates the imam’s actions, his prayer is invalid according to the sound view.
It is not necessary, however, to specify the particular imam by name.