Musar do Kapłańska 1:7

And one who issues a legal ruling in front of his teacher is liable. And our Rabbis, may their memory be blessed, said (Eruvin 63a) [that] when Nadav and Abihou brought a foreign fire, they did not die for the sin of bringing [it], since their intention was for the sake of a commandment: They said, “Behold it is written in the Torah (Leviticus 1:7), ‘And the sons of Aaron the priest shall put fire on the altar’ - although fire descends from Heaven, it is [nonetheless] a commandment to [also] bring ordinary fire.” Rather their punishment was because they issued a legal ruling in front of Moses, their teacher.