Rashi on Numbers 15:15/רש"י על במדבר ט״ו:ט״ו

1ככם כגר means, AS YOU ARE SO IS THE STRANGER (“you and the stranger shall be alike”). Thus (the use of the prefix כ with each of two things or persons stated to be exactly alike) is the Hebrew idiom to express complete similarity. Examples are: (Genesis 13:10) “As (כ) the garden of the Lord so (כ) is the land of Egypt”; (1 Kings 22:4) “As (כ) I am so (כ) art thou; as (כ) my people is so (כ) is thy people”.