Musannaf Ibn Abi Shayba 22:121

Abu Usama narrated to us, Sufyan narrated to me, he said: Aslam al-Minqari narrated to us, from Abdullah ibn Abd al-Rahman ibn Abza, from his father, from Ubayy, who said: The Book of Allah: whatever is clear to you from it, then act upon it. And whatever is ambiguous to you, then believe in it and entrust it to the one who knows it.