From Sahih Bukhari (Hadith 5994):
Narrated 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 (ﷺ) saying, "They (Hasan and Husain) are my two sweet-smelling flowers in this world
Reference: Bukhari 5994