Exodus 24:4

24:4 and Moses wrote down all the words of the Lord. Early in the morning he built an altar at the foot of the mountain and arranged twelve standing stones – according to the twelve tribes of Israel.

Exodus 28:21

28:21 The stones are to be for the names of the sons of Israel, twelve, according to the number of their names. Each name according to the twelve tribes is to be like the engravings of a seal.

Exodus 39:14

39:14 The stones were for the names of the sons of Israel, twelve, corresponding to the number of their names. Each name corresponding to one of the twelve tribes was like the engravings of a seal.