Adonai spoke to Moses saying,
It was on that very day that Adonai brought Bnei-Yisrael out of the land of Egypt as armies.
you are to set apart to Adonai every firstborn from the womb, and every firstborn male animal you have will be Adonai’s.
“Consecrate to Me all the firstborn, from every womb of Bnei-Yisrael, both men and animals—this is Mine.”