Numbers 10:7-9 New International Version - UK (NIVUK)
7 To 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. 9 When 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.