"40" is the 10th track on U2's 1983 album "War". Lyrically, the song is based on Psalm 40, straight out of the Bible. Below are the first 3 verses from Psalm 40, quoted from the New American Standard Version:

  1. I waited patiently for the Lord; And He inclined to me and heard my cry.
  2. He brought me up out of the pit of destruction, out of the miry clay, And he set my feet upon a rock making my footsteps firm.
  3. He put a new song in my mouth, a song of praise to our God; Many will see and fear and trust in the Lord.

Compare that to the lyrics of the actual song:

I waited patiently for the Lord
He inclined and heard my cry
He brought me up out of the pit
Out of the miry clay

I will sing, sing a new song
I will sing, sing a new song
How long to sing this song?
How long to sing this song?
How long, how long, how long
How long to sing this song?

You set my feet upon a rock
And made my footsteps firm
Many will see, many will see and hear

Legend has it that this song was recorded in the early morning hours and Bono's voice was starting to fade.

The first date on record that 40 was performed in concert is February 26, 1983. Throughout the 80's, U2 closed a large portion of their live shows with 40 and to date have performed it 351 times. With the 90's came a period in which U2 did not perform the song at all, but added the song back into their live setlist for 4 of the last performances on their Popmart tour in 1997 and 1998. For their Elevation tour in 2001, they performed the song multiple times. Time will tell whether or not they perform the song on their next tour.