Deuteronomy 21:16-17

then on the day when he wills his possessions to his sons, he cannot treat the son of his loved wife as firstborn in place of the son of the unloved wife—the [actual] firstborn. Instead he shall acknowledge the son of the unloved as the firstborn, by giving him a double portion of all that he has, for he was the beginning of his strength (generative power); to him belongs the right of the firstborn.