"All that open the womb are Mine, and every male firstborn among your livestock, whether ox or sheep.
Exodus 34:19
Surrounding context
34:18 "The Feast of Unleavened Bread you shall keep. Seven days you shall eat unleavened bread, as I commanded you, in the appointed time of the month of Abib; for in the month of Abib you came out from Egypt.
34:19 "All that open the womb are Mine, and every male firstborn among your livestock, whether ox or sheep.
34:20 But the firstborn of a donkey you shall redeem with a lamb. And if you will not redeem him, then you shall break his neck. All the firstborn of your sons you shall redeem. "And none shall appear before Me empty-handed.