“Then,” after safely crossing the Sea of Reeds, the Egyptians’ pursuit thwarted, “Moses and the Israelites sang…Miriam took her timbrel…and all the women followed her” (Exodus/Shemot 15:1, 20-21). “Then” — forty years later, after God tells Moses: “Assemble the people that I may give them water” (Numbers/Bamidbar 21:16) and without apparent prompting or leading — “Israel sang this song:”
Come up, O well — sing to it —
The well which the chieftans dug,
which the nobles of the people started
With maces, with their own staffs.
–Numbers/Bamidbar 21:17-18 (JPS translation**)
"A Song Every Day" 