Deuteronomy 21:16 VOICE

[16] he can't designate the eldest son of his favorite wife as the firstborn instead. When he divides his property and gives his sons their inheritances, he must recognize his true firstborn, the eldest son of the other wife, and give him a double portion of all his property as is customary for all men. That son was the first one created by the man's generative power, so the rights of the firstborn belong to him.