Another matter, “then Moses…sang [this song]” – that is what is written: “He raised me from the tumultuous pit,” (Psalms 40:3) – from Egypt. “From the miry mortar” (Psalms 40:3) – from mortar and bricks. “He set my feet upon a rock” (Psalms 40:3) – this is the sea. “He steadies my footsteps” (Psalms 40:3) – as they crossed on dry land.
Immediately, “he placed a new song in my mouth” (Psalms 40:4). This is analogous to a woman who was a menstruant. She completed her days of menstruation and became pure. She came to her husband, and he said to her: ‘Who attests that you are ritually pure?’
She said to him: ‘My maidservant will attest that I purified in her presence and I immersed.’ That is why it is stated: “This song,” – we are fit to recite song before you, as there is no impurity among us. The circumcision attests that we are pure; that is why it is stated: “This [hazot] song.” “This” means nothing other than circumcision, as it is stated: “This is my covenant that you shall observe…[circumcise every male]” (Genesis 17:10).
“And spoke, saying” (Exodus 15:1) – we will say to our children, and our children to their children, so they will recite a song like this before You when You perform miracles on their behalf.