Deuteronomy 21:16

Then it shall be, when he maketh his sons to inherit that which he hath,.... By a will in writing, or byword of mouth, or by a deed of gift, actually bestowing his goods upon them, and dividing among them what he is for the present possessed of; see Luke 15:12,

that he may not make the son of the beloved firstborn before the son of the hated, [which is] indeed the firstborn; that is, when such is the case, that the son of his wife he has the least value for is really his firstborn, he may not, through favour and affection to the wife he loves better, prefer her son, and declare him to be the firstborn, by devising to him or bestowing on him the double portion of his goods; for so to do would not be right, or agreeably to the will and law of God; for though previous to this law the birthright was given to Joseph, the eldest son of Rachel, the most beloved wife of Jacob, before Reuben who was the son of Leah, less beloved by him, and was in fact his firstborn; yet this was owing to the sin of Reuben, and by the appointment of God; see Genesis 49:3.