Deuteronomy 21:15
If a man have two wives, one beloved, and another hated, and they have born him children, the beloved and the hated; and the firstborn son be hers that was hated:
ā The Restored Word Bible
Context
13 And she shall put the raiment of her captivity from off her, and shall remain in your house, and bewail her father and her mother a full month: and after that you shall go in unto her, and be her husband, and she shall be your wife.
14 And it shall be, if you have no delight in her, then you shall let her go where she will; but you shall not sell her at all for money, you shall not make merchandise of her, because you hast humbled her.
15 If a man have two wives, one beloved, and another hated, and they have born him children, the beloved and the hated; and the firstborn son be hers that was hated:
16 Then it shall be, when he maketh his sons to inherit which he has, he may not make the son of the beloved firstborn before the son of the hated, the firstborn:
17 But he shall acknowledge the son of the hated the firstborn, by giving him a double portion of all that he has: for he the beginning of his strength; the right of the firstborn his.