Musannaf Ibn Abi Shayba 37:573

Abu Usama narrated to us, he said: Al-Thawri narrated to us, he said: Salim al-Minqari narrated to us, from Abdullah ibn Abd al-Rahman ibn Abza, from his father, who said: When what happened regarding the affair of Uthman occurred, and the people spoke about his matter, I came to Ubayy ibn Ka'b and said: "O Abu al-Mundhir, what is the way out?" He said: "The Book of Allah. What is made clear to you from it, act upon it and benefit from it. And what is ambiguous to you, believe in it and entrust it to the One who knows it."