Musannaf Ibn Abi Shayba 22:124

Ghundar narrated to us, from Shu'bah, from 'Amr ibn Murrah, from 'Abdullah ibn Salamah, from Mu'adh: That he said: "As for the Qur'an, it is like a lighthouse, like the lighthouse of a road, and it is not hidden from anyone. So, whatever you recognize of it, do not ask anyone about it. And whatever you are in doubt about, entrust it to one who knows."