Sahih Bukhari
Virtues and Merits of the Prophet (pbuh) and his Companions
Volume: 004 / Book Number: 056 / Hadith Number: 760
Narrated 'Aisha:
Whenever Allah's Apostle was given the choice of one of two matters, he would choose the easier of the two, as long as it was not sinful to do so, but if it was sinful to do so, he would not approach it. Allah's Apostle never took revenge (over anybody) for his own sake but (he did) only when Allah's Legal Bindings were outraged in which case he would take revenge for Allah's Sake.