Sahih Bukhari
Companions of the Prophet
Volume: 005 / Book Number: 057 / Hadith Number: 96
Narrated Ibn Abi Nu'm:
A person asked 'Abdullah bin 'Umar whether a Muslim could kill flies. I heard him saying (in reply). "The people of Iraq are asking about the killing of flies while they themselves murdered the son of the daughter of Allah's Apostle . The Prophet said, They (i.e. Hasan and Husain) are my two sweet basils in this world."