Another matter, “then Moses…sang” – that is what is written: “The Lord is known, He executed justice” (Psalms 9:17). It refers to the Egyptians, as the Holy One blessed be He executed justice against them in Egypt and at the sea. Rabbi Yehoshua says: They were afflicted with ten plagues with one finger, as it is stated: “The magicians said to Pharaoh: It is the finger of God” (Exodus 8:15). At the sea they were afflicted with fifty plagues, as it is stated: “Israel saw the great power [hayad hagedola]” (Exodus 14:31).12The literal translation of hayad hagedola is “the great hand.”
The hand has five fingers, and five times ten are fifty. Likewise, you find with Job that he was afflicted with fifty plagues, as it is stated: “Pity me, pity me; you are my friends, as the hand of God has touched me” (Job 19:21). Rabbi Eliezer says: The Egyptians were afflicted with forty plagues in Egypt and two hundred at the sea, as each and every plague that came upon them, they would come as four, as it is stated: “He destroyed their vines with hail” (Psalms 78:47), and it is written: “[He sent upon them] His burning anger; rage, fury, trouble, a band of evil angels” (Psalms 78:49).
“Rage” is one; “fury” is two, “trouble” is three; “a band of evil angels” is four. That is, with a finger they were afflicted with forty plagues and with a hand two hundred. That is, “He executed justice.” What is, “the Lord is known”?
It is as it is written: “Egypt will know that I am the Lord,” (Exodus 14:18), when I impose punishment upon Pharaoh, as it is written afterward: “When I am glorified through Pharaoh” (Exodus 14:18). And it is written: “With the work of his hands, the wicked is snared” (Psalms 9:17) – this is Pharaoh. What did he say? “Every son that is born [you shall cast him into the Nile]” (Exodus 1:22).
The Holy One blessed be He, too, did to him as he had done, as it is stated: “He tossed Pharaoh and his host in the Red Sea” (Psalms 136:15). What is: “Higayon Selah” (Psalms 9:17)? Israel said: What is incumbent upon us to do? It is only to recite songs and paeans, as it is stated: “Then Moses…sang [yashir].”13The term higayon is thus interpreted as referring to song [nigun] and thought (as in Psalms 19:15).
Selah means forever (Eruvin 54a). Thus, the Israelites realized that they should sing to God forever (Midrash HaMevoar). Then Moses and the children of Israel sang [yashiru] is not stated, but rather, “then Moses sang [yashir].”14The term yashir is singular, whereas the plural term is yashiru. Rabbi Yehoshua ben Levi said: From here, the passage was sung by Moses, and then our ancestors sang it at the sea.