Musannaf Ibn Abi Shayba 35:440

Waki' narrated to us, from Hisham ibn 'Urwah, from his father, from Zaynab bint Umm Salamah, from Umm Salamah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "You bring your disputes to me, and I am only a human being. It may be that some of you are more eloquent in their argument than others, and I only judge between you according to what I hear from you. So, whoever I judge in his favor regarding any of his brother's rights, let him not take it, for I am only cutting out for him a piece of fire which he will bring on the Day of Resurrection."