Sahih Bukhari

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 Messenger (ﷺ) . The Prophet (ﷺ) said, They (i.e. Hasan and Husain) are my two sweet basils in this world