Halakha Ulangan 23:9: Mishneh Torah, Marriage

There are some intercourse prohibitions [that are inferred] from a positive commandment and are not among the negative prohibitions. There are three of them: a first or second generation male or female Egyptian or Edomite, and a non-virgin to a high priest. Since it is not written regarding these that “one may not come” or “one may not take”, but rather [concerning an Egyptian or Edomite] “they may join the congregation of God in the third generation” (Deuteronomy 23:9), you may infer that first and second generation [Egyptians and Edomites] are forbidden to join [the congregation of God]; and since it is stated [concerning a high priest] “he must marry a virgin” (Leviticus 21:13) you may infer that [a high priest] may not marry a non-virgin. And a negative prohibition that is inferred from a positive commandment has the status of a positive commandment.