Musnad Ahmad 32:1093

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, he said: Ma'mar narrated to me, from Qatadah, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "There will be disagreement and division in my nation. A people will emerge from among them who recite the Quran, but it will not pass beyond their collarbones. Their mark is shaving and *tasbit*." *Tasbit* means: the complete removal of short hair. "So when you see them, put them to sleep [i.e., kill them]."