Sunan Abi Dawud, Book 37, Hadith 4317 (USC-MSA) ⓘ

Muhammad ibn al-Munkadir told that he saw Jabir ibn Abdullah swearing by Allah that Ibn as-Sa'id was the Dajjal (Antichrist). I expressed my surprise by saying: You swear by Allah! He said: I heard Umar swearing to that in the presence of the Messenger of Allah, but the Messenger of Allah did not make any objection to it.

حَدَّثَنَا ابْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، عَنْ سَعْدِ بْنِ إِبْرَاهِيمَ، عَنْ مُحَمَّدِ بْنِ الْمُنْكَدِرِ، قَالَ رَأَيْتُ جَابِرَ بْنَ عَبْدِ اللَّهِ يَحْلِفُ بِاللَّهِ أَنَّ ابْنَ صَائِدٍ الدَّجَّالُ، فَقُلْتُ تَحْلِفُ بِاللَّهِ فَقَالَ إِنِّي سَمِعْتُ عُمَرَ يَحْلِفُ عَلَى ذَلِكَ عِنْدَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَلَمْ يُنْكِرْهُ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏.‏