Musannaf Ibn Abi Shayba 22:13

Yahya ibn Adam narrated to us, he said: Hammad ibn Zayd narrated to us, from Yazid ibn Hazim, from Sulayman ibn Yasar, he said: Umar reached a group of people who were reciting to one another. When they saw Umar, they fell silent. He said: "What were you reviewing?" We said: "We were reciting to one another." He said: "Recite, and do not commit grammatical errors (lahn)."