Êxodo 28:10

כתולדתם ACCORDING TO THEIR BIRTH — in the order in which they were born: Reuben, Simeon, Levi, Judah, Dan, Naphtali upon one, and upon the other Gad, Asher, Issachar, Zebulun, Joseph and Benjamin — the last name being written full (plene, with a י before the last letter, thus: בנימין) for that is how it is written in the passage that tells of his birth (Genesis 35:18). This gives twenty-five letters on each stone (cf. Sotah 36a).

שִׁשָּׁה֙ מִשְּׁמֹתָ֔ם עַ֖ל הָאֶ֣בֶן הָאֶחָ֑ת וְאֶת־שְׁמ֞וֹת הַשִּׁשָּׁ֧ה הַנּוֹתָרִ֛ים עַל־הָאֶ֥בֶן הַשֵּׁנִ֖ית כְּתוֹלְדֹתָֽם׃

six of their names on one stone, and the other six names on the other stone, according to their birth.