Sunan Ibn Majah, Chapter: The Book of the Sunnah

"The Messenger of Allah (saw) said: 'At the end of time there will appear a people with new teeth (i.e., young in age), with foolish minds. They will speak the best words ever uttered by mankind and they will recite the Qur'an, but it will not go any deeper than their collarbones. They will pass through Islam like an arrow passes through its target. Whoever meets them, let him kill them, for killing them will bring a reward from Allah for those who kill them." (Sahih)