Midrash su Salmi 47:5

Rabbi Joshua, son of Ḳorchah, said: Forty days was Moses on the mountain, reading the Written Law by day, and studying the Oral Law by night. After the forty days he took the tables (of the Law) and descended into the camp on the 17th of Tammuz, and he broke in pieces the tables, and slew the sinners in Israel. He then spent forty days in the camp, until he had burnt the calf, and powdered it like the dust of the earth, and he had destroyed the idol worship from Israel, and he instituted every tribe in its place. And on the New Moon of Ellul the Holy One, blessed be He, said to him: "Come up || to me on the mount" (Ex. 24:12), and let them sound the Shophar (trumpet) throughout the camp, for, behold, Moses has ascended the mount, so that they do not go astray again after the worship of idols. The Holy One, blessed be He, was exalted with that Shophar, as it is said, "God is exalted with a shout, the Lord with the sound of a trumpet" (Ps. 47:5). Therefore the sages instituted that the Shophar should be sounded on the New Moon of Ellul every year.