I saw Jabir bin Abdullah swearing by Allah that Ibn al-Sayyad was the Dajjal.I said, "You swear by Allah?"He replied, "I heard Umar swearing to that in the presence of the Prophet ﷺ, and the Prophet ﷺ did not object to it."
رأيت جابر بن عبد الله يحلف بالله أن ابن الصياد الدجال. قلت: تحلف بالله؟ قال: إني سمعت عمر يحلف على ذلك عند النبي ﷺ، فلم ينكره النبي ﷺ.
Bukhari related it in "Holding Fast" (7355) and Muslim in "Tribulations" (2929), both through Ubaydullah bin Muaz Anbari, who said, my father narrated to us, Shubah narrated to us, from Saad bin Ibrahim, from Muhammad bin Munkadir, who mentioned it.