Deuteronomy 21:15-17 - The Right Of The Firstborn
15 If a man have two wives and loves one more than the other. The first son of either wife is the man's first-born son, even if the boy's mother is the wife he does not love. A man near death and dividing up his property must give a double share to his first-born son. This is because he was the first to be born. 16 The day he wills his possessions to his sons, he should not put the firstborn son of the loved wife before the firstborn son of the unloved wife. 17 He should acknowledge the firstborn son of the unloved wife. And give him a double portion of all that he has. He is the beginning of his strength. The right of the firstborn belongs to him.