Numbers 10:21

And the LORD spake unto Moses, saying,

Make thee two trumpets of silver; of a whole piece shalt thou make them: that thou mayest use them for the calling of the assembly, and for the journeying of the camps.

And when they shall blow with them, all the assembly shall assemble themselves to thee at the door of the tabernacle of the congregation.

And if they blow but with one trumpet, then the princes, which are heads of the thousands of Israel, shall gather themselves unto thee.