Musnad Ahmad 27:1946

Sulayman ibn Dawud narrated to us, Shu'bah informed us, from Muhammad ibn Abi Ya'qub, who said: I heard Ibn Abi Nu'm saying: I witnessed Ibn 'Umar when a man from the people of Iraq asked him about a person in the state of ihram who killed a fly. He said: "O people of Iraq, you ask me about a person in the state of ihram who killed a fly, while you have killed the son of the daughter of the Messenger of Allah, peace and blessings of Allah be upon him? And the Messenger of Allah, peace and blessings of Allah be upon him, said: 'They are my two sweet basils from the world.'"