Musnad Abu Ya'la al-Mawsili 1:59

Musa ibn Muhammad ibn Hayyan narrated to us, [who said] Rawh ibn Aslam and Fahd narrated to us, they said: Abd al-Aziz ibn Muslim narrated to us, [who said] Layth narrated to us, from Abu Muhammad, from Ma’qil ibn Yasar, who said: I witnessed the Prophet, peace and blessings of Allah be upon him, with Abu Bakr—or he said: Abu Bakr narrated to me from the Prophet, peace and blessings of Allah be upon him—that he said: "Shirk (polytheism) is more hidden among you than the crawling of an ant." Abu Bakr said: "And is shirk anything other than calling upon another god alongside Allah?" The Messenger of Allah, peace and blessings of Allah be upon him, said: "Shirk is more hidden among you than the crawling of an ant. Shall I not guide you to that which will remove from you its small and its great [parts]? Say: 'O Allah, I seek refuge in You from associating anything with You while I know, and I seek Your forgiveness for what I do not know.'"