Musnad Ahmad 27:1119

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Muhammad ibn Abi Ya'qub, who said: I heard Ibn Abi Nu'm, who said: I heard Abdullah ibn Umar ibn al-Khattab, and a man asked him about something—Shu'bah said: And I think he asked him about a person in the state of ihram killing a fly—so Abdullah said: "The people of Iraq ask about a fly, yet they have killed the son of the daughter of the Messenger of Allah, may Allah bless him and grant him peace, while the Messenger of Allah, may Allah bless him and grant him peace, said: 'They are my two sweet basils from the world.'"