Harun was Musa's elder brother and an eloquent speaker. Before going to Pharaoh, Musa asked Allah: give me my brother Harun from my family as a helper; strengthen me through him and make him share my task, so that we may glorify You much and remember You much. Allah said: your request is granted.
The two brothers went together to Pharaoh's court. Allah had taught them: speak to him gently; perhaps he will take heed or fear.
The calf
After crossing the sea, Musa went up Mount Tur for forty nights, leaving Harun in charge of the people. Meanwhile a man called as-Samiri melted down ornaments and made the figure of a calf that gave out a sound. Many said: this is your god!
Harun did not stay silent. He warned them:
But they said: we will not stop worshipping it until Musa returns to us.
Musa returned and, seeing this, in anger and grief seized his brother by the beard and head: when you saw them going astray, why did you not come after me? Harun answered calmly:
Musa understood, and asked Allah for forgiveness and mercy for himself and his brother.