Midrash su Salmi 135:9

"and I shall smite": I might think, through an angel or through a messenger; it is, therefore, written (Ibid. 29) "and the L rd (Himself) smote every first-born" — not through an angel or a messenger. "and I smote every first born": even from different places. Whence do I derive (the same) even for the first-born of Egypt who were in other places? (From Psalms 136:10) "He smote Egypt through their first-born" (connoting, even if they were not in Egypt). Whence do I derive (the same for) the first-born of Cham and Cush? (viz. Genesis 10:6) From (Psalms 78:51) "And He struck every first-born in Egypt, the first fruit of their strength in the tents of Cham." "from man until beast": The initiator of the transgression was struck first. Similarly, (Genesis 7:23) "And all that existed on the face of the earth was blotted out — from man to beast, etc." Similarly, (Exodus 14:4) "And I will be honored through (the downfall of) Pharaoh and his entire host." Similarly, (Devarim 13:16) "Smite the inhabitants of that city … and its cattle." Similarly, (Numbers 5:27) "and her belly ('first in the transgression') will swell, and her thigh will fall." Here, too, "And I shall smite every first-born in the land of Egypt from man until beast." The initiator of the transgression was struck first. Now does this not follow a fortiori, viz.: If in His measure of punishment — the lesser (measure) — the initiator of the transgression is smitten first, how much more so in His measure of beneficence — the greater (measure) — (is the initiator of the mitzvah rewarded first)!