Rashi on Numbers 3:29/רש"י על במדבר ג׳:כ״ט

1משפחת בני קהת יוחנו … תימנה THE FAMILY OF THE SONS OF KOHATH SHALL ENCAMP … SOUTHWARD — and next to them was the division of Reuben who, too, were encamped in the South (cf. Numbers 2:10) — “Woe to the wicked, woe to his neighbour!” It was on this account (on account of the friendship caused by this proximity) that some of them (the Reubenites) — Dathan and Abiram and 250 men (who were also mostly Reubenites, cf. Rashi on Numbers 16:1) — were punished together with Korah and his congregation, (cf. Numbers 16:32 ff.), because they were dragged into the quarrel with them (cf. Midrash Tanchuma, Vayera 12).