Charles Ellicott Commentary Exodus 13:12

"that thou shalt set apart unto Jehovah all that openeth the womb, and every firstling which thou hast that cometh of a beast; the males shall be Jehovah`s." — Exodus 13:12 (ASV)

You shall set apart—that is, to separate it from the rest of the flock or herd, so that it would not be mixed with those that were not "sanctified."