Musnad Ahmad 25:1185

'Affan narrated to us, Wuhayb narrated to us, Dawud narrated to us, from 'Ikrimah, from Ibn 'Abbas, who said: "Abu Jahl came to the Prophet, may Allah bless him and grant him peace, while he was praying, and he forbade him. The Prophet, may Allah bless him and grant him peace, threatened him. He said: 'Do you threaten me? By Allah, I am the one with the largest assembly in the valley.' Then Allah revealed: 'Have you seen the one who forbids a servant when he prays? Have you seen if he is upon guidance or enjoins righteousness? Have you seen if he denies and turns away?' [Al-'Alaq: 9-13]. Ibn 'Abbas said: 'By the One in Whose hand is my soul, if he had called his assembly, the angels of punishment would have seized him.'"