Leviticus 10:1
1 Then Aaron’s sons, Nadab and Abihu, each took his fire pan and put fire in it, set incense on it, and presented strange fire before the Lord, which he had not commanded them to do. 2 So fire went out from the presence of the Lord and consumed them so that they died before the Lord. 3 Moses then said to Aaron, “This is what the Lord spoke: ‘Among the ones close to me I will show myself holy, and in the presence of all the people I will be honored.’” So Aaron kept silent.
Numbers 26:61
59 Now the name of Amram’s wife was Jochebed, daughter of Levi, who was born to Levi in Egypt. And to Amram she bore Aaron, Moses, and Miriam their sister. 60 And to Aaron were born Nadab and Abihu, Eleazar and Ithamar. 61 But Nadab and Abihu died when they offered strange fire before the Lord. 62 Those of the Levites who were numbered were 23,000, all males from a month old and upward, for they were not numbered among the Israelites; no inheritance was given to them among the Israelites. 63 These are those who were numbered by Moses and Eleazar the priest, who numbered the Israelites in the rift valley plains of Moab along the Jordan River opposite Jericho.
Notes and References
"... Leviticus 10 presents Nadab and Abihu in a cultic setting. Each took his fire-holder and, placing fire in it, laid incense over it. Leviticus 10:1 notes that they offered before Yahweh “alien fire”, which they were not ordered to offer. J. Milgrom lists several explanations for the phrase “alien fire.” First, some suggest that in the accounts of Nadab and Abihu and Korah and his band, the phrase “each his fire-holder” (Leviticus 10:1; Numbers 16:17-18) refers to private pans. Yet the interpretation that Nadab and Abihu were punished because they used private pans rather than utensils of the sanctuary (Leviticus 16:12; Numbers 16:46) should be ruled out since “alien” describes the fire rather than the fire-holder. Another possibility is to take “fire” as designating coals. Instead of being taken from the outer altar (Leviticus 16:12; Numbers 16:46), the “alien [unauthorized] coals” come from a source that is “profane” (Targum Onqelos on Leviticus 16:1), “outside” (Jerusalem Targum) or from an oven (Targum Pseudo-Jonathan) ... In the way it is told, this old story invalidates all incense offerings outside the sanctuary, which, based on the archaeological evidence dating from the First and Second Temple periods, were quite prevalent and persistent in the religious life of Israel. Numbers 3:4 adds three details concerning the tragic fate of Aaron’s sons: they were killed “while offering alien fire” (compare Numbers 26:61); this mysterious episode occurred in the “wilderness of Sinai”; and since Nadab and Abihu had no sons, Eleazar and Ithamar, their brothers, continued their duty as priests under Aaron’s guidance ..."
Alexander, T. Desmond, and David W. Baker Dictionary of the Old Testament: Pentateuch (pp. 583-584) InterVarsity Press, 2003