Musannaf Ibn Abi Shayba 22:3

'Isa ibn Yunus narrated to us, from Thawr, from 'Amr ibn Dinar, who said: 'Umar wrote to Abu Musa: "To proceed: Gain understanding of the Sunnah, and gain understanding of the Arabic language, and make the Quran clear (i.e., recite it with correct grammar), for it is Arabic. And adopt the ways of Ma'add (i.e., the lifestyle of the desert Arabs), for you are of the people of Ma'add."