Rashi on Numbers 16:8/רש"י על במדבר ט״ז:ח׳

1ויאמר משה אל קרח שמעו נא בני לוי AND MOSES SAID UNTO KORAH, HEAR, I PRAY YOU, YE SONS OF LEVI — He began to speak to him with gentle words; when, however, he saw that he was stubborn, he said to himself: Before the other tribes join him and perish together with him, I will speak to them all!” He thereupon began to admonish them: “Hear ye, ye sons of Levi!” (Midrash Tanchuma, Korach 6).