Musnad Abu Ya'la al-Mawsili 155:301

Muhammad ibn Bashar narrated to us, Ibn Abi ‘Adi and ‘Abd al-Rahman narrated to us, from Shu‘bah, who said: Mansur wrote it to me, and I read it to him. He said: Abu ‘Uthman, the freed slave of al-Mughirah ibn Shu‘bah, narrated to me, saying: I heard Abu Hurayrah saying: I heard the Messenger of Allah (peace and blessings of Allah be upon him)—the master of this chamber, the truthful, the believed—saying: "Mercy is not snatched away except from a wretched person."