Numbers 10:4-9

4If only one is sounded, the leaders—the heads of the clans of Israel—are to assemble before you. 5When a trumpet blast is sounded, the tribes camping on the east are to set out. 6At the sounding of a second blast, the camps on the south are to set out. The blast will be the signal for setting out.

7To gather the assembly, blow the trumpets, but not with the signal for setting out. 8“The sons of Aaron, the priests, are to blow the trumpets. This is to be a lasting ordinance for you and the generations to come. 9When you go into battle in your own land against an enemy who is oppressing you, sound a blast on the trumpets. Then you will be remembered by the LORD your God and rescued from your enemies.