Joshua 22:27-28, 34

but that it may be a witness between you and us and our generations after us, that we may perform the service of the LORD before Him with our burnt offerings, with our sacrifices, and with our peace offerings; that your descendants may not say to our descendants in time to come, “You have no part in the LORD.” ’ Therefore we said that it will be, when they say this to us or to our generations in time to come, that we may say, ‘Here is the replica of the altar of the LORD which our fathers made, though not for burnt offerings nor for sacrifices; but it is a witness between you and us.’

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.”