Musnad Ahmad 788:122

Abdullah ibn Numayr narrated to us, Abd al-Malik—meaning Ibn Abi Sulayman al-Arzami—narrated to us, from Abu Ali, a man from Banu Kahil, who said: Abu Musa al-Ash'ari delivered a sermon to us and said: "O people, beware of this shirk (polytheism), for it is more hidden than the crawling of an ant." Abdullah ibn Huzn and Qays ibn al-Mudarib stood up to him and said: "By Allah, you shall either retract what you said, or we will go to Umar, whether we are granted permission or not." He said: "Rather, I will retract what I said. The Messenger of Allah (peace and blessings of Allah be upon him) delivered a sermon to us one day and said: 'O people, beware of this shirk, for it is more hidden than the crawling of an ant.'" Then someone whom Allah willed to speak said to him: "And how do we beware of it, while it is more hidden than the crawling 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.'"