Musa asked Allah to make his eloquent brother Harun his helper, and Allah granted it (20:29–36). When Musa went to the appointment with his Lord, he left Harun in charge of their people (7:142).
While Musa was away, some of the people worshipped the calf. Harun warned them but they would not listen; he explained to Musa that he feared causing division among them (20:90–94).