Numbers 21:17

Then Israel sang this song
Being affected with the free favour and good will of God towards them:

spring up, O well;
for the springing up of which they prayed in faith, believing in the promise of God, that it would spring up; and so encouraged one another not only to believe it, but even to sing on account of it before it actually did:

sing ye unto it;
or on account of it praise the Lord for it; or "answer to it" ^F13, it being their manner to sing their songs by responses, or alternately.