Rashi on Numbers 27:3/רש"י על במדבר כ״ז:ג׳

1והוא לא היה וגו׳ AND HE WAS NOT [… IN THE CONGREGATION OF KORAH] — Because they intended to state בחטתו מת, that HE DIED IN HIS OWN SIN they felt compelled to say he had taken no part in the sin of those who murmured, nor had he been in the congregation of Korah who incited the people against the Holy One, blessed be He (cf. Sifrei Bamidbar 133:3, Bava Batra 118b), but he had died through his own sin only, and had not made others to sin with him (Sifrei Bamidbar 133:3). — As regards what this sin was, R. Akiba said that he was the man who gathered sticks on the Sabbath day (Numbers 15:32); R. Simeon said that he was one of those who presumed to disobey God’s command (Numbers 14:44) (Shabbat 96b).