Exodus 13:15

And it came to pass, when Pharaoh would hardly let us go,.... Showed great reluctance to it, and with difficulty was prevailed upon to dismiss them:

that the Lord slew all the firstborn in the land of Egypt, both the firstborn of men, and the firstborn of beast: which he did in one night, making use of a destroying angel or angels for that purpose:

therefore I sacrifice to the Lord all that openeth thee matrix, being males; that is, the firstborn of all clean creatures, as oxen, sheep, and goats:

but all the firstborn of my children I redeem; by paying five shekels apiece to the priest for them, as before observed; and this law continues to be observed with the Jews; the manner of which, as related by Leo Modena i, is as follows,

"Thirty days being expired after the birth of the child, they call a priest to them; that is to say, one that is descended of the stock of Aaron, whom the father of the child pleaseth; and so, many people being gathered together at the time appointed, the father of the child bringeth before the priest, in a bowl or basin, a good quantity of gold and silver, and then they give him the child into his arms; the priest then calling the mother of it before him, saith unto her, mistress, is this your son? she answereth, yes; then, replies he, have you never had any child before, either male or female, or have miscarried anyone? she saith unto him, no; then doth the priest say, this child is mine, as being the firstborn; then turning himself toward the father, he asketh him, whether he will redeem it or not? who answereth him, saying, see, here is gold and silver, take your own price; then saith the priest unto him, you will redeem it then? the father answereth, I will redeem it; it shall be so then, saith the priest, this child is mine as being the firstborn, as it is written, Numbers 18:16. I therefore take this in exchange, c. and so he takes the sum of two French crowns, or thereabout, as he thinks good, and then delivers the child to his father and mother, and this day they make a feasting day.''

This custom was used in Christ's time, and was observed with respect to him, Luke 2:27.

i History of the Jews, par. 4. c. 9. sect. 2.