Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 6284
Abu Huraira reported it was said to Allah's Messenger (may peace be upon him): Invoke curse upon the polytheists, whereupon he said: I have not been sent as the invoker of curse, but" I have been sent as mercy."