Another matter, “then Moses…sang” – that is what is written: “She opens her mouth with wisdom and the teaching of kindness is on her tongue” (Proverbs 31:26). From the day that the Holy One blessed be He created the world until Israel stood at the sea, we do not find a person who sang a song to the Holy One blessed be He other than Israel. He created Adam, the first man, and he did not recite song.
He rescued Abraham from the fiery furnace and from the kings, and he did not recite song. Likewise, [He rescued] Isaac from the knife and he did not recite song. Likewise, [He rescued] Jacob from the angel, from Esau, and from the residents of Shekhem, and he did not recite song. When Israel came to the sea and it was split for them, they immediately sang a song before the Holy One blessed be He, as it is stated: “Then Moses and the children of Israel sang.”
That is, “she opens her mouth with wisdom.” The Holy One blessed be He said: “I have been waiting for [people such as] these [to recite song].” Az means nothing other than rejoicing, as it is stated: “Then [az] our mouth will be filled with laughter” (Psalms 126:2). Rabbi Yehuda ben Pazi said: What did Israel see, that led them to recite song with az?
They said: ‘Initially, this sea was dry land, and the generation of Enosh arose and incensed Him with az, as it is stated: “Then [az] they began profaning the name of the Lord” (Genesis 4:26). The Holy One blessed be He rendered it [dry land] sea and exacted retribution against them, as it is stated: “Who calls the waters of the sea and pours them on the face of the earth” (Amos 5:8; 9:6). Now it was sea, and it was transformed to dry land on our behalf, as it is stated: “The children of Israel went on dry land in the midst of the sea” (Exodus 14:29).
We will laud him with az because he transformed the sea into dry land; that is, “then Moses and the children of Israel sang.” Az means nothing other than security, as it is stated: “Then [az] you shall walk on your path securely” (Proverbs 3:23).