Bilangan 3:13

3:13 because all the firstborn are mine. When I destroyed ¹  all the firstborn in the land of Egypt, I set apart for myself all the firstborn in Israel, both man and beast. They belong to me. I am the Lord.” ² 

Bilangan 31:30

31:30 From the Israelites’ half-share you are to take one portion out of fifty of the people, the cattle, the donkeys, and the sheep – from every kind of animal – and you are to give them to the Levites, who are responsible for the care of the Lord’s tabernacle.”

Bilangan 31:47

31:47 From the Israelites’ share Moses took one of every fifty people and animals and gave them to the Levites who were responsible for the care of the Lord’s tabernacle, just as the Lord commanded Moses.

[3:13]  ¹ tn The form הַכֹּתִי (hakkoti) is the Hiphil infinitive construct of the verb נָכָה (nakhah, “to strike, smite, attack”). Here, after the idiomatic “in the day of,” the form functions in an adverbial clause of time – “when I destroyed.”

[3:13]  ² sn In the Exodus event of the Passover night the principle of substitution was presented. The firstborn child was redeemed by the blood of the Lamb and so belonged to God, but then God chose the Levites to serve in the place of the firstborn. The ritual of consecrating the firstborn son to the Lord was nevertheless carried out, even with Jesus, the firstborn son of Mary (Luke 2:22-23).