(Until, when he reached the setting place of the sun,) means, he followed a route until "he reached the furthest point that could be reached in the direction of the sun's setting, which is the west of the earth." As for the idea of his reaching the place in the sky where the sun sets, this is something impossible, and the tales told by storytellers that he traveled so far to the west that the sun set behind him are not true at all. Most of these stories come from the myths of the People of the Book and Christians and the fabrications and lies of their heretics.