Rashi on Numbers 10:5/רש"י על במדבר י׳:ה׳

1ותקעתם תרועה WHEN YE BLOW AN ALARM — The signal for the camps to set out on the journey was a Tekiah, a Teruah (a tremulous sound) and again a Tekiah. Thus is it derived in the Siphre from the apparently redundant verses (Sifrei Bamidbar 73:2; cf. Rosh Hashanah 34a).