Numbers 10:5

“Make two trumpets of silver. You shall make them of beaten work. You shall use them for the calling of the congregation and for the journeying of the camps.

When they blow them, all the congregation shall gather themselves to you at the door of the Tent of Meeting.

If they blow just one, then the princes, the heads of the thousands of Israel, shall gather themselves to you.

When you blow an alarm the second time, the camps that lie on the south side shall go forward. They shall blow an alarm for their journeys.

But when the assembly is to be gathered together, you shall blow, but you shall not sound an alarm.

“The sons of Aaron, the priests, shall blow the trumpets. This shall be to you for a statute forever throughout your generations.