Musnad Ahmad 31:295

Waki’ narrated to us, ‘Ikrimah ibn ‘Ammar narrated to us, from ‘Asim ibn Shumaykh, from Abu Sa’id al-Khudri, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would swear an oath and exert himself in the oath, he would say: "No, by the One in Whose hand is the soul of Abu al-Qasim, a people will emerge from my nation whose deeds you will deem insignificant compared to their deeds. They will recite the Quran, but it will not pass beyond their collarbones. They will pass through Islam just as an arrow passes through the game." They said: "Is there a sign by which they are known?" He said: "Among them is a man with a deformed hand—or a breast—and they will have their heads shaved." Abu Sa’id said: "Twenty—or twenty-something—of the companions of the Prophet (peace and blessings of Allah be upon him) narrated to me that ‘Ali (may Allah be pleased with him) was the one who killed them." He said: I saw Abu Sa’id after he had grown old, and his hands were trembling, saying: "Fighting them is more lawful to me than fighting the same number of Turks."