Deuteronomy 21:15-16

15A man may have two wives, but he may love only one of them. Each wife may give birth to a son. The firstborn son may be the son of the wife that he does not love.

16But when the man shares his things with his children, he must promise this: After his death, the firstborn son will receive what he should have. The man may not choose to give the double share of his things to the son of the wife that he loves.