Musnad Ahmad 30:883

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Mansur, from Abu ‘Uthman, from Abu Hurayrah, who said: I heard the Messenger of Allah—the truthful, the believed, Abu al-Qasim, the master of the chamber, may Allah bless him and grant him peace—saying: "Mercy is not snatched away except from a wretched person." Shu‘bah said: "He wrote it to me, and I read it to him," meaning Mansur.