C. F. Keil and Franz Delitzsch on Exodus 7:4-7

Exo 7:4-7 את־ידי ונתתּי: “I will lay My hand on Egypt,” i.e., smite Egypt, “and bring out My armies, My people, the children of Israel.” צבאות (armies) is used of Israel, with reference to its leaving Egypt equipped (Exo 13:18) and organized as an army according to the tribes (cf. Exo 6:26 and Exo 12:51 with Num 1 and 2), to contend for the cause of the Lord, and fight the battles of Jehovah. In this respect the Israelites were called the hosts of Jehovah. The calling of Moses and Aaron was now concluded. Exo 7:6 and Exo 7:7 pave the way for the account of their performance of the duties consequent upon their call.