Musnad Ahmad 1034:108

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Amr ibn Murrah, from Abu al-Bakhtari al-Ta’i, from Abu Thawr, who said: ‘Uthman sent Sa‘id ibn al-‘As on the day of al-Jur‘ah. He said: So they went out to him and turned him back. He said: I was sitting with Abu Mas‘ud and Hudhayfah. Abu Mas‘ud said: "I did not think that he would return without blood being shed in it." Hudhayfah said: "But I knew that it would surely turn back on its heels without a cupping-glass worth of blood being shed in it. And I did not know anything of that except something I learned while Muhammad, peace and blessings of Allah be upon him, was alive: that a man wakes up a believer, then reaches the evening with nothing of it left with him, and he reaches the evening a believer, and wakes up with nothing of it left with him. He fights for his faction today, and Allah kills him tomorrow. His heart is inverted, and his backside rises above his front." He said: I said: "His lower part?" He said: "His backside."