Midras Mazmur 136:10: Midrash Tanchuma Buber, Mekhilta DeRabbi Yishmael, Shemot Rabbah & Pirkei DeRabbi Eliezer

(Deut. 2:31:) MOREOVER, THE LORD SAID {UNTO MOSES} [UNTO ME]: SEE I HAVE BEGUN TO GIVE SIHON AND HIS LAND OVER TO YOU. [It is also written] (in Amos 2:9): YET I DESTROYED THE AMORITE¹²Although THE AMORITE implies the Amorite people in the biblical context, in the context of the midrash the singular AMORITE refers to Sihon. [BEFORE THEM, WHOSE STATURE WAS LIKE THE CEDARS IN HEIGHT]. By virtue of what? By virtue of the Torah, which the sages had taught. Our masters have said: Sihon was strong.¹³Below, Deut. 1a:8; M. Pss. 136:11. His height was like a wall tower, and he was stronger than all creatures. He was taller than any tower on earth, but his feet reached to the earth. So no creature was able to stand before him, just as it says (ibid., cont.): YET I DESTROYED HIS FRUIT ABOVE <AND HIS ROOTS BELOW>. What did the Holy One do? He bound the <angelic> prince that belonged to him and to his land. Then he cast him {in} [from] his place and handed him over to Israel. Our masters have said: Sihon and Og were stronger than Pharaoh and his armies; for just as they uttered a song over the fall of Pharaoh, so were they worthy to utter a song at their fall. It is simply that David came and uttered a song over them, as stated (in Ps. 136:10): TO THE ONE WHO SMOTE EGYPT THROUGH THEIR FIRST-BORN…. (vss. 17, 19) TO THE ONE WHO SMOTE GREAT KINGS …; SIHON, KING OF THE AMORITES….

"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).

"and the L–rd smote every first-born in the land of Egypt": even (those first-born) from different places. And whence is the same derived for the first-born of Egypt (who were found) in different places? From (Psalms 136:10) "Who smote Egypt by their first-born" (wherever they were). Whence do I derive the same for the first-born of Cham, Kush, Put, and Lud? From (Ibid. 78:51) "He smote every first-born in Egypt, the first-fruit of their strength in the tents of Cham."