Musnad Ahmad 1156:146

Waki’ narrated to us, he said: Hisham ibn ‘Urwah narrated to us, 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. So, whoever I judge in his favor regarding something of his brother’s right, let him not take it, for I am only cutting out for him a piece of the Fire which he will bring with him on the Day of Resurrection."