Musannaf Ibn Abi Shayba 22:4

Yahya ibn Adam narrated to us, he said: Hammad ibn Zayd narrated to us, he said: Wasil, the freed slave of Uyaynah, narrated to us, from Yahya ibn Aqil, from Yahya ibn Ya'mar, from Ubayy ibn Ka'b, who said: "Learn the Arabic language just as you learn the memorization of the Quran."