Sifrei Devarim 216

¹(Devarim 21:16) "Then it shall be on the day that he causes his sons to inherit": Inheritances are adjudicated in the daytime and not at night. ²"what there shall be to him": We are hereby taught that the son inherits what is anticipated as well as what is on hand. ³"he shall not grant primogeniture": We are hereby taught that he is not permitted to grant it. I might think that he is not permitted to do so, but if he did so, it stands; it is, therefore, written "he shall not be able to grant primogeniture," (the implication being that) if he does, it shall not stand. ⁴"… over the face of the son of the hated one": We are hereby taught that if his head or most of it emerged live (from the womb), he negates the next birth as a first-born. ⁵"...in disregard of the son of the unloved one who is older": Although the older is of the unloved one.