Musnad Ahmad 27:1225

Abu al-Nadr narrated to us, Mahdi narrated to us, from Muhammad ibn Abi Ya'qub, from Ibn Abi Nu'm, who said: A man came to Ibn Umar while I was sitting, and he asked him about the blood of a mosquito. He said to him: "Where are you from?" He said: "From the people of Iraq." He said: "Look at this man! He asks about the blood of a mosquito, yet they have killed the son of the Messenger of Allah (peace and blessings of Allah be upon him), and I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'They are my two sweet basils from the world.'"