Sahih Muslim Book 4, Hadith Number 1437.

Asim reported: I asked Anas whether Qunut was observed (by the Holy prophet) before ruku’ or after ruku’. He replied: Before ruku’. I said: People conceive that the Messenger of Allah (may peace be upon him) observed Qunut after the ruku’. He said: The Messenger of Allah (may peace be upon him) observed Qunut (after the ruku’ as the people conceive it) for a mouth invoking curse upon those persons who had killed men among his Companions who were called the reciter (of the Qur’an).