Abu Sa'id al-Raqashi (may Allah have mercy on him) narrates: I went in to the Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), and she asked: "What is the matter with Abu al-Hasan (Sayyiduna Ali, may Allah be pleased with him) that he is killing his own companions, the Qur'an reciters (scholars of the Qur'an)?" He said: I said: "O Mother of the Believers! We found Dhu al-Thudayyah (the man with a breast-like lump of flesh) among the slain." On hearing this she heaved a deep sigh (or took a long breath), then said: "The one who conceals testimony is just like the one who gives false testimony; I heard the Messenger of Allah (peace and blessings be upon him) say: 'This group will be killed by the best person of my Ummah.'"
This hadith is narrated from Abu Sa'id al-Raqashi (may Allah have mercy on him) only with this chain, and Muhammad bin al-Muthanna (may Allah have mercy on him) is alone in narrating it.