Musnad Ahmad 30:1953

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah narrated to us, from Abu Yunus, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Allah, I take a covenant with You that You will not break with me. I am only a human being, so whichever servant I have whipped, or insulted, or cursed, make it for him a prayer and a means of drawing near [to You]."