Judges 21:4 (KJV)

And the people came to the house of God, and abode there till even before God, and lifted up their voices, and wept sore;

And said, O Lord God of Israel, why is this come to pass in Israel, that there should be to day one tribe lacking in Israel?

And it came to pass on the morrow, that the people rose early, and built there an altar, and offered burnt offerings and peace offerings.

And the children of Israel said, Who is there among all the tribes of Israel that came not up with the congregation unto the Lord? For they had made a great oath concerning him that came not up to the Lord to Mizpeh, saying, He shall surely be put to death.

And the children of Israel repented them for Benjamin their brother, and said, There is one tribe cut off from Israel this day.