When Musa was called by Allah to face Pharaoh, he felt his tongue was tied — he had a slight stutter from childhood. Musa prayed: "My Lord, expand my chest, ease my task, untie the knot from my tongue. Make Harun, my brother, my helper." (Taha: 25–30)
Allah granted it. Harun, who was fluent in speech, was made a prophet and sent to accompany Musa. Together they faced Pharaoh.
When Musa went to Mount Sinai to receive the Torah (40 days), he entrusted the Children of Israel to Harun: "Take my place among my people. Reform, and don't follow the path of corruptors."
But during that time, a man named Samiri made a golden calf and claimed it was the Children of Israel's god. Many followed in worship. Harun tried to counsel: "You are only being tested. Your real Lord is the Most Merciful. Follow me."
Most refused. They threatened Harun. When Musa returned and grew angry, Harun said: "O son of my mother, don't grab my beard, don't pull my hair. I feared you would say 'you've divided the Children of Israel.'"
Musa realized Harun was right — he chose to hold the people together without bloodshed rather than act harshly.