Midrash su Numeri 10:7

(Bamidbar 10:7) "And when the assembly is to be gathered, you shall blow a tekiah, but not a teruah.": What is the intent of this? Because it is written (Ibid. 2) "And they (the trumpets) shall be for you for convoking the congregation and for the traveling of the camps," just as the convoking of the congregation is with two (trumpets), viz. (Ibid. 3) "And when they blow with them, etc.," so, the traveling of the camps is with two. I might then think that just as the traveling of the camps is with tekiah-teruah-tekiah, so, the convoking of the congregation; it is, therefore, written "And when the assembly is to be gathered, you shall blow a tekiah, but not a teruah."