All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and messenger.
The Hadeeth which you mentioned in the question was classified by some scholars, like Al-Haakim and Ath-Thahabi, as Saheeh (sound). As-San’aani said: "'Umar narrated that the Prophet said: “A man should not be asked as to why he has beaten his wife.” [Abu Daawood, An-Nasaa-i, Ibn Maajah, and Al-Haakim classified it as authentic and Ath-Thahabi agreed with him. As-Suyooti pointed out that this Hadeeth is Saheeh." [Excerpted from Fat-h Al-Ghaffaar Al-Jaami’ li Ahkaam Sunnat Nabiyyina Al-Mukhtaar 3/1491]
Al-Bayhaqi reported the Hadeeth with the following wording: “Do not ask a man why he has beaten his wife, and do not sleep except after having offered Witr prayer.” [As-Sunnan Al-Kubra by al-Bayhaqi (7/497)]
However, some scholars classified this Hadeeth as Dha'eef (weak) such as Shaykh Ahmad Shaakir and Shaykh Al-Albaani .
Shaykh Al-Albaani said: "... I said: This is a weak chain of narrators because among the narrators is Al-Musliyy (i.e. Abdurrahmaan Al-Musliyy who narrated this Hadeeth from Al-Asha'th ibn Qays on the authority of 'Umar ibn Al-Khattaab from the Prophet )." Ath-Thahabi said: "He is not known except by this Hadeeth, and the only one who narrated from him was Daawood ibn 'Abdullah Al-Awdi. The Haafidh [i.e. Ibn Hajar] said: “He is acceptable." Note: Ath-Thahabi did not comment on this Hadeeth in his Mukhtasar, so Shaykh Ahmad Shaakir commented on this in his Commentary, and he said after attributing it to Musnad Ahmad, "Its chain of narrators is weak, as the narration by Daawood ibn 'Abdullah Al-Awdi is weak, so the fact that Al-Munthiri kept quiet about it (without criticizing it) is negligence on his part.
And I (Al-Albaani) say: "Daawood ibn 'Abdullah Al-Awdi is a trustworthy narrator and I think that Shaykh Ahmad Shaakir mistakenly thought that he was Daawood ibn Yazeed Al-Awdi, uncle of ‘Abdullaah ibn Idrees, who is a weak narrator, and he was not the one who narrated this Hadeeth, even though Shaykh Ahmad understood it to be narrated by him in his commentary on Al-Musnad." [Irwaa’ al-Ghaleel fi Takhreej Ahadeeth Manaar as-Sabeel (7/99)]
Finally, for the Ahaadeeth about which the scholars differed in regard to their authenticity, the layman should follow concerning them the opinion of whoever he considers to be most trustworthy in regard to his knowledge and religion among the scholars who are specialized in this knowledge [i.e. in sciences of Hadeeth].
For more information, please refer to Fataawa 83741 and 34815.
Allaah Knows best.