Joshua 3:3 (BSB)

and commanded the people: “When you see the ark of the covenant of the LORD your God being carried by the Levitical priests, you are to set out from your positions and follow it.

From Joshua 3. Also in the ESV.

Commentary on Joshua 3:3

• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on Joshua 3:1-6: Rahab, in mentioning to the spies the drying up of the Red Sea (Jos 2:10), the report of which terrified the Canaanites more than anything else, intimates that those on that side the water expected that Jordan, that great defence of their country, would in like manner give way to them. Whether the Israelites had any expectation of it does not appear.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on Joshua 3:3: And they commanded the people,.... In the name of Joshua, by whom they were sent: saying, when ye see the ark of the covenant of the Lord your God, and the priests the Levites bearing it: the Targum reads, the priests and Levites; so the Septuagint: it was the business of the Levites, particularly the Kohathites, to bear the ark in journeying, but here the...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on Joshua 3:3: When ye see the ark . . ., and the priests the Levites bearing it--The usual position of the ark, when at rest, was in the center of the camp; and, during a march, in the middle of the procession.
• Keil & Delitzsch (Lutheran), Biblical Commentary on the Old Testament on Joshua 3:1-4: Jos 3:1-4 “Arrangements for the Passage through the Jordan. - When they reached the Jordan, the Israelites rested till they passed over. לוּן, to pass the night; then in a wider sense to tarry, Pro 15:31; here it means to rest. According to Jos 3:2, they stayed there three days.