Deuteronomy 21:16-17

then it shall be, on the day he bequeaths his possessions to his sons, that he must not bestow firstborn status on the son of the loved wife in preference to the son of the unloved, the true firstborn. But he shall acknowledge the son of the unloved wife as the firstborn by giving him a double portion of all that he has, for he is the beginning of his strength; the right of the firstborn is his.