Musnad Ahmad 33:1177

'Ali ibn Bahr narrated to us, 'Isa ibn Yunus narrated to us, al-A'mash narrated to us, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Allah, I am only a human being, so whichever man among the Muslims I have insulted, or lashed, or cursed, make it for him a purification and a reward."