C. F. Keil and Franz Delitzsch on Judges 7:4

Jdg 7:4

But even this number was regarded by the Lord as still too great, so that He gave to Gideon the still further command, “Bring them (the 10,000 men) down to the water,” i.e., the waters formed from the fountain of Harod, “and I will purify them for thee there (צרף, separate those appointed for the battle from the rest of the army; the singular suffix refers to העם), and say to thee, This shall go with thee, and that,” i.e., show thee each individual who is to go with thee to the battle, and who not.