Rashi on Exodus 28:10/רש"י על שמות כ״ח:י׳

1כתולדתם 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).