Musannaf Ibn Abi Shayba 3:1234

Sharīk narrated to us, from ‘Alī ibn Zayd, from Zurārah ibn Awfā, who said: Abū Mūsā read to me a letter from ‘Umar to him: "Recite to the people during the ‘Ishā’ prayer from the middle of the Mufaṣṣal."