Musannaf Ibn Abi Shayba 3:4584

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from Abu Malik al-Ashja'i, who said: I said to my father, "O my father, you prayed behind the Prophet, may Allah bless him and grant him peace, and behind Abu Bakr, Umar, and Uthman. Did you see any of them perform Qunut?" He said, "O my son, it is an innovation."