(29) Excellent Qualities and Description of the Prophet
كتاب الفضائل والشمائل
(3a) Chapter: His Character and Natural Disposition - Section 1
(3) باب في أخلاقه وشمائله صلى الله عليه وسلم - الفصل الأول
Mishkat al-Masabih 5812 
Abu Huraira told that when God's messenger was asked to invoke a curse on the polytheists, he replied, "I was not sent as one given to cursing; I was sent only as a mercy." Muslim transmitted it.
وَعَنْ أَبِي هُرَيْرَةَ قَالَ: قِيلَ: يَا رَسُولَ اللَّهِ ادْعُ عَلَى الْمُشْرِكِينَ. قَالَ: «إِنِّي لَمْ أُبْعَثْ لَعَّانًا وَإِنَّمَا بُعِثْتُ رَحْمَةً» . رَوَاهُ مُسْلِمٌ
Reference : Mishkat al-Masabih 5812
In-book reference : Book 29, Hadith 72