Musannaf Ibn Abi Shayba 33:1489

Muhammad ibn Abd Allah ibn al-Zubayr narrated to us, he said: Abd al-Aziz ibn Abi Rawwad narrated to us, from Abd Allah ibn Ubayd ibn Umayr, he said: It used to be said: Knowledge is the lost property of the believer; he goes out in the morning in search of it, and when he finds something of it, he gathers it.