Musnad Ahmad

It is narrated from Hazrat Abu Saeed Khudri (may Allah be pleased with him) that when the Prophet (peace and blessings be upon him) would swear a very firm oath about something, he would say, "No, by Him in Whose hand is the soul of Abu al-Qasim." On one occasion, after swearing such an oath, he said, "A people will surely appear in my Ummah whose deeds you will consider insignificant compared to your own deeds. They will recite the Qur'an, but it will not go beyond their throats. They will leave Islam just as an arrow passes through its prey." The Companions asked, "Is there any sign by which they can be recognized?" The Prophet (peace and blessings be upon him) said, "Among them will be a man on whose hand there will be a mark like a woman's breast, and their heads will be shaved." Hazrat Abu Saeed (may Allah be pleased with him) says that even more Companions have told me that Hazrat Ali (may Allah be pleased with him) killed those people. The narrator says that I saw Hazrat Abu Saeed Khudri (may Allah be pleased with him) in his old age when his hands were trembling, and he was saying that in my view, killing those people was more lawful than killing so many Turks.