Sahih Bukhari Volume 5, Book 57, Hadith Number 96.

Narated By 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.”