He swore by Allah that Ibn Sayyad was the Dajjal.I asked, "You swear by Allah?"He replied, "I heard Umar swear to that in the presence of the Messenger of Allah ﷺ, and the Messenger of Allah ﷺ did not object to it."
حدثنا ابن معاذ حدثنا أبي حدثنا شعبة عن سعد بن إبراهيم عن محمد بن المنكدر قال
يحلف بالله أن ابن صائد الدجال فقلت تحلف بالله فقال إني سمعت عمر يحلف على ذلك عند رسول الله ﷺ فلم ينكره رسول الله ﷺ
Muslim narrated it in the chapter on Fitan, section on the mention of Ibn Sayad, hadith 2929/94, and Bukhari in the chapter on Holding Fast to the Book and the Sunnah, section on those who considered the Prophet's ﷺ not condemning something to be evidence, etc., hadith 7355, from the hadith of Ubaydullah bin Muaz, through this same chain.