Musnad Abu Ya'la al-Mawsili 209:116

Abu Khaythama narrated to us, Yahya ibn Sa'id narrated to us, from Hisham, who said: My father informed me, from Zaynab bint Umm Salama, from Umm Salama, from the Prophet (may Allah bless him and grant him peace), who said: "You bring your disputes to me, and perhaps one of you is more eloquent in his argument than another. I am only a human being. So, whoever I judge in his favor regarding any part of his brother's right, I am only cutting out for him a piece of the Fire, so let him not take it."