Rashi on Numbers 10:4/רש"י על במדבר י׳:ד׳

1ואם באחת יתקעו AND IF THEY BLOW BUT WITH ONE TRUMPET (i.e. a “Tekiah” on one trumpet) — it is a sign to summon the princes, as it is said here, “then the princes shall gather themselves unto thee”. Their meeting place, too, was at the entrance of the tent of meeting. This is derived from a ג"ש in the Sifrei (Sifrei Bamidbar 73:1 from the use in both cases of the same expression ותקעו and יתקעו: not from ונועדו — ונועדו as erroneously stated.)