Sahih Bukhari Volume 001, Book 003, Hadith Number 085.
Narated By Abu Huraira : The Prophet said, "(Religious) knowledge will be taken away (by the death of religious scholars) ignorance (in religion) and afflictions will appear; and Harj will increase." It was asked, "What is Harj, O Allah's Apostle?" He replied by beckoning with his hand indicating "killing." (Fateh-al-Bari Page 192, Vol. 1)