Lévitique 10:1

But the sons of Aharon, Nadab and Abihu, took each man his censer, and put fire therein, and laid sweet incense upon it, and offered before the Lord strange fire taken from (under) the hearth-pots, which had not been commanded them.

his coal-pan each one took his own coal-pan. In my opinion, this event also occurred “on the eighth day”; and this is borne out later on: “…they had already sacrificed their sin-offering…today” [:19].

put fire in it this fire was not taken from the fire that had descended from God [9:24] — it was “unconsecrated fire”.

which He had not commanded them both the burning of the incense and the use of unconsecrated fire were their own idea, not a commandment.

ויקחו בני אהרן נדב ואביהוא, before the fire had emanated from the Holy of Holies they each took his own pan planning to offer incense on the golden altar, seeing that this offering precedes (timewise) the disposal of not totally burned leftovers from the previous day’s sacrificial meat. They placed “alien” fire on these pans, something which had not been commanded by Moses to be done on this day. Even though on ordinary days the rule of “the sons of Aaron will place in these pans fire on the altar” (Leviticus 1,7) was in effect, this rule did not apply to the day of inauguration, and Moses had not wanted any man made fire to be introduced into the Tabernacle. This was because he expected heavenly fire to manifest itself so that the addition of man made fire would have completely ruined the impact of the miracle. On this day the name of G’d would be glorified by all the people becoming aware of Divine approval of their offerings, by G’d accepting same by means of heavenly fire We find a parallel to this in Kings I 18,25 when the prophet Elijah on Mount Carmel ordered the priests not to put any fire on the altar.

ויקחו שני בני אהרן איש מחתתו, they thought that just as after the daily communal offering which was accompanied by the manifestation of G’d’s Presence as we know from Exodus 29,42 עולת תמיד לדורותיכם פתח אהל מועד לפני ה' אשר איועד לכם שמה, “the regular communal burnt offerings for your generations in the presence of the Lord, where I will manifest Myself there,” there would come the incense, so it would also be in order on this occasion to present a new incense offering honouring the manifestation of the Lord to the whole people and in honour of the heavenly fire having descended.

This is why they offered it in the sanctuary, i.e. 'לפני ה, on the golden altar. The Torah had stipulated that no foreign incense [nor other unauthorised offerings. Ed.] was to be offered on that altar (Exodus 30,9). Even assuming that the golden altar would qualify for additional incense offerings when a specific command to do so would be issued, the sons of Aaron sinned by doing this now and not having consulted with their mentors. This is why the Torah stresses

אשר לא צוה אותם, “which He had not commanded them (to do).”

וישימו עליה קטורת, “and they placed incense thereon;” they placed the incense of the fire. We never find that incense was placed directly on the censer.

אש זרה אשר לא צוה אות, “strange fire which He had not commanded them (to use).” The Torah implies that they had already been warned not to use such fire as is written in Exodus 30,9: [Actually, in that verse, the prohibition is not to offer strange fire, but not to use a strange mixture of incense. Ed.] The reason the Torah uses the unusual formulation “which he had not commanded them,” instead of ”which He had forbidden them,” is that on occasion, in exceptional circumstances, G–d does permit something which is ordinarily prohibited. The Torah underlines that this was not such an occasion. An example of such an exception would be what Elijah did on Mount Carmel in his confrontation with the 400 hundred priests of the Baal. (Kings I 18). He used a private altar something which was strictly forbidden since the Temple had been erected. We also find a verse in Jeremiah 7,31, where the prophet uses this expression as having been used by G–d. In that example the prophet chides the people for sacrificing their children to G–d saying that this had never occurred for G–d to have them do. (Compare Talmud, tractate Taanit, folio 4 on that subject)