Musnad Ahmad 2:41

Sulayman ibn Dawud—meaning Abu Dawud al-Tayalisi—narrated to us, he said: Abu 'Awanah narrated to us, from Dawud al-Awdi, from 'Abd al-Rahman al-Musli, from al-Ash'ath ibn Qays, who said: I was a guest of 'Umar, and he reached for his wife and struck her. Then he said: "O Ash'ath, memorize from me three things that I memorized from the Messenger of Allah (peace and blessings of Allah be upon him): Do not ask a man why he struck his wife, and do not sleep unless you have performed Witr." And I forgot the third one.