Sahih Bukhari Volume 3, Book 43, Hadith Number 638.

Narated By Um Salama : (The wife of the Prophet) Allah’s Apostle heard some people quarreling at the door of his dwelling. He came out and said, “I am only a human being, and opponents come to me (to settle their problems); maybe someone amongst you can present his case more eloquently than the other, whereby I may consider him true and give a verdict in his favor. So, If I give the right of a Muslim to another by mistake, then it is really a portion of (Hell) Fire, he has the option to take or give up (before the Day of Resurrection).”