Numbers 5:4

And the children of Israel did so, and put them without the
camp
Aben Ezra observes, that this was done immediately before they journeyed, and that those that were defiled journeyed between the standard of Ephraim and the standard of Dan; but this, he says, was by way of conjecture, since it is not expressed:

as the Lord spake unto Moses, so did the children of Israel;
they were obedient in this particular.