Abu Jahl passed by the Messenger of Allah (peace be upon him) on Mount As-Safa, and harmed and swore at him. But the Messenger of Allah (peace be upon him) did not reply to him at all. The slave girl of Abdullah ibn Judan heard this incident from her living quarters. When Abu Jahl left the Messenger of Allah (peace be upon him), he went to a group of Quraysh who were at the Kaaba and sat with them.
Soon after, Hamzah ibn Abdul-Muttalib (may Allah be pleased with him) came, carrying a bow as he was returning from a hunting trip. It was the habit of his to circumumbulate the Kaaba after returning from a hunting trip before going to his family. When Hamzah passed by the slave girl, after the Messenger of Allah (peace be upon him) had returned to his home, she told Hamzah, “Oh Abu Amara, if you only saw what Abu Al-Hakim ibn Hesham did to your nephew Muhammed!” And she told him what had happened.
Hamzah became very angry, and so he left heading for Abu Jahl to strike him when he saw him. When Hamzah entered the mosque, he saw Abu Jahl sitting with some people. Hamzah went to him, raised his bow and hit him with it. Voicing his strong disapproval of what Abu Jahl had done, Hamzah said, “Do you swear at him and I follow his religion and agree with what he says?”
“So reply to my hitting you, if you can.” One man from Banu Mukhzoom stood up and went to Hamzah in order to defend Abu Jahl, but Abu Jahl said, “Leave Abu Amara, as, by Allah, I swore at his nephew using strong words.”
After that, Hamzah (may Allah be pleased with him) remained in Islam, and used to do whatever the Messenger of Allah (peace be upon him) told the Muslims to do. When Hamzah embraced Islam, Quraysh realized that the Messenger of Allah (peace be upon him) had gained power and support, and that Hamzah would protect him. Therefore, the harm the Muslims suffered from Abu Jahl lessened [The Prophet’s Biography by Ibn Hesham].