I said to Ibn Umar , "What about a man who performs the lian oath against his wife?"He replied, "The Messenger of Allah ﷺ separated them," and he then mentioned the rest of the hadith.
حدثنا سفيان عن أيوب
قلت لابن عمر رضي الله عنه رجل لاعن امرأته فقال فرق رسول الله ﷺ بينهما وذكر الحديث
Its isnad is sahih. Ayub is Ibn Sakhtiyani, Saeed is Ibn Jubayr, and the hadith will come again at 4477, 4950, and see also 4693. These seven hadiths, 392–398, are not from the Musnad of Umar, as you can see. The first of them is from the Musnad of Anas bin Malik, and the rest are from the Musnad of Abdullah bin Umar.