Êxodo 21:4 Midrash: Mekhilta DeRabbi Yishmael, Sifra & Shemot Rabbah

(Exodus 21:3) "If alone (i.e., without a wife) he came, alone shall he go out." What is the intent of this? It is written (9) "If his master gives him a woman" (a Canaanite bondswoman). ("If" here is) optional (and not mandatory). You say "optional," but perhaps it is mandatory. It is, therefore, written "If alone he come, alone shall he go out" — It is optional and not mandatory. These are the words of R. Yishmael.

(Exodus, Ibid.) "If he were the husband of a woman": Scripture speaks of a Jewish woman. You say "a Jewish woman," bur perhaps a gentile woman is intended? (This cannot be, for) (4) "If his master gives him a woman" speaks of a Canaanite woman. How, then, is "If he were the husband of a woman" to be understood? As referring to a Jewish woman.

(Exodus 21:4) "If his master gives him a wife": ("If" here connotes) optionality. But perhaps it is mandatory? (This is not so, for) "If alone (i.e., without a wife) he came, alone shall he go out" connotes what is optional and not what is mandatory. These are the words of R. Yishmael.