Musannaf Ibn Abi Shayba 3:1217

Sharīk narrated to us, from ‘Alī ibn Zayd, from Zurārah ibn Awfā, who said: Abū Mūsā read to me the letter of ‘Umar: "That you should recite to the people in the Maghrib [prayer] from the end of the Mufaṣṣal."