Deuteronomy 21:15 CEV

[15] Moses said to Israel: Suppose a man has 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 the man doesn't love. Later, when the man is near death and is dividing up his property, he must give a double share to his first-born son, simply because he was the first to be born.