It is narrated from Ash'ath bin Qays (may Allah be pleased with him); he said: I was a guest of Sayyiduna Umar bin al-Khattab (may Allah be pleased with him). He got up during part of the night and struck his wife. Then he called me: "O Ash'ath!" I said: "At your service." He said: "Memorize from me three things which I memorized from the Messenger of Allah (peace and blessings be upon him): 'Do not ask a man why he strikes his wife; do not ask him about those of his brothers (friends) on whom he relies or does not rely; and do not sleep until you have prayed witr.'" This hadith is sahih al-isnad (its chain is authentic), but the two Shaykhs (Bukhari and Muslim) did not record it.