From Sahih Muslim (Hadith 1549):
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 (ﷺ) observed Qunut after the ruku'. He said: The Messenger of Allah (ﷺ) 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)
Reference: Muslim 1549