Musnad Abu Ya'la al-Mawsili 1:58

Musa ibn Muhammad ibn Hayyan narrated to us, [he said] Rawh ibn Aslam and Fahd narrated to us, they said: Abd al-Aziz ibn Muslim narrated to us, [he said] Layth narrated to us, from Abu Muhammad, from Ma'qil ibn Yasar, who said: I witnessed the Prophet, may Allah bless him and grant him peace, with Abu Bakr—or he said: Abu Bakr narrated to me from the Prophet, may Allah bless him and grant him peace—that he said: "Shirk (polytheism) is more hidden among you than the crawling of an ant." Then he said: "Shall I not guide you to that which, if you say it, will take away from you the small of it and the great of it? 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.'"