Joshua 22:10, 34

And when they came to the region of the Jordan which is in the land of Canaan, the children of Reuben, the children of Gad, and half the tribe of Manasseh built an altar there by the Jordan—a great, impressive altar.

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