Sahih Bukhari Volume 8, Book 73, Hadith Number 23.

Narated By Ibn Abi Na’m : I was present when a man asked Ibn ‘Umar about the blood of mosquitoes. Ibn Umar said, “From where are you?” The man replied. “From Iraq.” Ibn ‘Umar said, “Look at that! he is asking me about the blood of Mosquitoes while they (the Iraqis) have killed the (grand) son of the Prophet. I have heard the Prophet saying, “They (Hasan and Husain) are my two sweet-smelling flowers in this world.”