Ibn Abi Nu'm said, "I was with Ibn 'Umar when a man asked him about the blood of gnats. He asked, 'Where are you from?' 'From the people of Iraq,' he replied. He said, 'Look at this man! He asks about the blood of gnats when they murdered the grandson of the Prophet, may Allah bless him and grant him peace! I heard the Prophet, may Allah bless him and grant him peace, say, 'They are my sweet basil in this world.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُوسَى، قَالَ: حَدَّثَنَا مَهْدِيُّ بْنُ مَيْمُونٍ، قَالَ: حَدَّثَنَا ابْنُ أَبِي يَعْقُوبَ، عَنِ ابْنِ أَبِي نُعْمٍ قَالَ: كُنْتُ شَاهِدًا ابْنَ عُمَرَ إِذْ سَأَلَهُ رَجُلٌ عَنْ دَمِ الْبَعُوضَةِ؟ فَقَالَ: مِمَّنْ أَنْتَ؟ فَقَالَ: مِنْ أَهْلِ الْعِرَاقِ، فَقَالَ: انْظُرُوا إِلَى هَذَا، يَسْأَلُنِي عَنْ دَمِ الْبَعُوضَةِ، وَقَدْ قَتَلُوا ابْنَ النَّبِيِّ صلى الله عليه وسلم، سَمِعْتُ النَّبِيَّ صلى الله عليه وسلم يَقُولُ: هُمَا رَيْحَانَيَّ مِنَ الدُّنْيَا.
sunnah.com