Deuteronomy 21:16

when that man assigns his inheritance to his sons he must not appoint the son of the beloved wife as the firstborn over the son of the unloved wife.

Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn:

then it shall be, in the day that he causes his sons to inherit that which he has, that he may not give the son of the beloved the rights of the firstborn before the son of the hated, who is the firstborn;

then it shall be, in the day that he causeth his sons to inherit that which he hath, that he may not make the son of the beloved the first-born before the son of the hated, who is the first-born: