Exodus 13:15
And it came to pass, when Pharaoh would hardly let us go, that the Lord slew all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of beast: therefore I sacrifice to the Lord all that openeth the matrix, being males; but all the firstborn of my children I redeem.
Cross-references
Exodus 13:12 gives the command to consecrate firstborns, which 13:15 then explains with the exodus event.
Exodus 12:29 narrates the actual plague on firstborns that 13:15 references as the reason for consecration.
Exodus 7:16 records God's command through Moses to let His people go, showing the original refusal that led to the plague.
Exodus 11:5 prophesies the death of firstborns, which 13:15 refers to as the reason for the redemption practice.
Exodus 34:20 repeats the redemption of firstborns, reinforcing the same practice explained in 13:15.
Exodus 12:26 parallels the son's question about Passover, while 13:15 answers the similar question about firstborns.
Numbers 3:13 repeats this same claim — the firstborn belong to God because He struck down Egypt's firstborn.
Numbers 8:17 similarly states that the firstborn of Israel are consecrated to God from that day.
Psalm 78:51 directly recounts God striking the firstborn of Egypt — the very event that grounds the command in Exodus 13:15.
Psalm 135:8 explicitly mentions striking the firstborn of Egypt, both man and beast — the same event as in Exodus 13:15.