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

حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا غُنْدَرٌ، حَدَّثَنَا شُعْبَةُ، عَنْ مُحَمَّدِ بْنِ أَبِي يَعْقُوبَ، سَمِعْتُ ابْنَ أَبِي نُعْمٍ، سَمِعْتُ عَبْدَ اللَّهِ بْنَ عُمَرَ، وَسَأَلَهُ، عَنِ الْمُحْرِمِ،، قَالَ شُعْبَةُ أَحْسِبُهُ يَقْتُلُ الذُّبَابَ فَقَالَ أَهْلُ الْعِرَاقِ يَسْأَلُونَ عَنِ الذُّبَابِ وَقَدْ قَتَلُوا ابْنَ ابْنَةِ رَسُولِ اللَّهِ ﷺ، وَقَالَ النَّبِيُّ ﷺ " هُمَا رَيْحَانَتَاىَ مِنَ الدُّنْيَا ".