Musnad Ahmad 539:3

'Affan narrated to us, Shu'bah narrated to us, he said: Yazid ibn Khumayr informed me, he said: I heard Shurahbil ibn Shuf'ah narrating from 'Amr ibn al-'As: That the plague occurred, and 'Amr ibn al-'As said: "It is an impurity (rijs), so disperse from it." And Shurahbil ibn Hasanah said: "I have accompanied the Messenger of Allah, peace and blessings of Allah be upon him, and 'Amr is more astray than the camel of his family"—and perhaps Shu'bah said: "more astray than the camel of his family"—"and he (the Prophet) said: 'It is a mercy from your Lord, and the supplication of your Prophet, and the death of the righteous before you, so gather and do not disperse from it.'" He said: That reached 'Amr ibn al-'As, and he said: "He has spoken the truth."