Musannaf Ibn Abi Shayba 33:870

Waki’ narrated to us, from ‘Ubayd Allah ibn Mawhab, from Salih ibn Sa’id the Mu’adhdhin, who said: While I was with ‘Umar ibn ‘Abd al-‘Aziz in al-Suwayda’, I gave the adhan for the final Isha prayer. He prayed, then entered the palace. He had barely stayed long when he came out and prayed two light rak’ahs. Then he sat and wrapped his arms around his legs (ihtaba), and began reciting [Surah] al-Anfal. He did not cease repeating it, and whenever he passed by a verse of warning, he supplicated humbly, and whenever he passed by a verse of mercy, he prayed, until I gave the adhan for the dawn prayer.