I saw Jabir bin Abdullah swearing by Allah that Ibn al-Sayyad was the Dajjal.I said, "Do 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 الصائد الدجال، قلت: أتحلف بالله؟ قال: إني سمعت عمر يحلف على ذلك عند النبي ﷺ فلم ينكره النبي ﷺ.
Bukhari narrated it in Kitab al-Itisam (7355), and Muslim in Kitab al-Fitan wa Ashrat as-Saah (2929), both from the hadith of Ubaydullah bin Muaz, who said, my father narrated to us, Shubah narrated to us, from Saad bin Ibrahim, from Muhammad bin Munkadir, and he mentioned it.Abu Dawud (4330) narrated with a sahih chain from Nafi, who said, Ibn Umar used to say, "By Allah, I have no doubt that the Messiah Dajjal is Ibn Sayad."Regarding his statement, "that Ibn Sayad is the Dajjal," Sheikh Islam bin Taymiya said, "The matter of Ibn Sayad was confusing to some of the companions, so they thought he was the Dajjal, and the Prophet ﷺ held back judgment about him until it later became clear to him that he was not the Dajjal, but rather he was of the type of soothsayers who have satanic states. That is why ﷺ used to go to him to test him." The Distinction between the Allies of the Merciful and the Allies of Satan (p. 77).I say, it appears from this that he was born bearing a great resemblance to the Dajjal in terms of physical form and characteristics. That is why the Prophet did not find it objectionable that he might be the Dajjal, had he been presented with that possibility, as in the hadith of Abu Saeed Khudri. And because such statements and states came from him, his matter became confusing to some of the companions.