Sahih Bukhari Volume 003, Book 049, Hadith Number 868.
Narated By 'Aisha : Once Allah's Apostle heard the loud voices of some opponents quarreling at the door. One of them was appealing to the other to deduct his debt and asking him to be lenient but the other was saying, "By Allah I will not do so." Allah's Apostle went out to them and said, "Who is the one who was swearing by Allah that he would not do a favor?" That man said, "I am that person, O Allah's Apostle! I will give my opponent whatever he wishes."