Rashi on Numbers 10:7/רש"י על במדבר י׳:ז׳

1ובהקהיל את הקהל וגו׳ AND WHEN THE CONGREGATION IS TO BE GATHERED TOGETHER etc. — Because it is stated, (v. 2) “and they shall be unto thee for calling together the assembly and for the journeying of the camps”, you must reason as follows: How is it in the case of “the calling of the assembly”? A Tekiah was made by two priests and on both of them (on two trumpets), since it states, (v. 3) “when they blow with them (cf. Rashi on that verse). So, too, is the setting out of the camps to be by means of both trumpets. One might, however, also reason as follows: How is it in the case of the “journeyings of the camps”? The priest sounds a Tekiah, a Teruah, and another Tekiah (cf. Rashi on v. 5)! So, too, in the case of “calling the assembly together”, he blows a Tekiah, a Teruah and a Tekiah, so that it follows that there is no difference between the signal for “calling the assembly” and that for the “journeyings of the camps”. Scripture, however, states here: “And when the congregation is to be gathered together, etc.” to intimate that no Teruah is sounded at the calling of the assembly (ולא תריעו), and the same rule applies to calling the princes. — Thus we have a different signal for the three occasions: “the calling of the assembly” is done by both trumpets, and that of the princes by one — but neither in this nor that signal is there a Teruah, whilst “the journeying of the camps” is done by both trumpets with a Teruah, a Tekiah preceding and following it (Sifrei Bamidbar 74).