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 (ie Hasan and Husain) are my two sweet Basils in this world".
The hadith also serves as a reminder for us not to forget what happened in Iraq when people killed the son of the daughter of Allah's Messenger (ﷺ). We must remember that we should never do anything wrong or hurtful towards anyone, even if it is someone we don't know very well or don't like very much.