Musnad Ahmad 30:1949

Aswad narrated to us, Israel narrated to us, from al-A’mash, 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 Muslim I have cursed or harmed, make it for him a purification and a means of drawing near [to You]."