Joshua 22:10, 34

And when they came to the region of the Jordan that is in the land of Canaan, the people of Reuben and the people of Gad and the half-tribe of Manasseh built there an altar by the Jordan, an altar of imposing size.

The people of Reuben and the people of Gad called the altar Witness, “For,” they said, “it is a witness between us that the LORD is God.”