Musannaf Ibn Abi Shayba 21:427

Abdullah ibn Numayr narrated to us, Abd al-Malik ibn Abi Sulayman narrated to us, from Abu Ali, a man from Banu Kahil, who said: Abu Musa al-Ash'ari addressed us and said: The Messenger of Allah (may Allah bless him and grant him peace) addressed us one day and said: "O people, beware of this shirk (polytheism), for it is more hidden than the creeping of an ant." Someone who wished to speak said to him: "And how do we beware of it while it is more hidden than the creeping of an ant, O Messenger of Allah?" He said: "Say: 'O Allah, we seek refuge in You from associating anything with You that we know, and we seek Your forgiveness for what we do not know.'"