'Muhammad b. Munkadir reported:As I saw Jabir b. 'Abdullah taking an oath in the name of Allah that it was Ibn Sa'id who was the Dajjal I said: Do you take an oath in the name of Allah? Thereupon he said: I heard 'Umar taking an oath in the presence of Allah's Apostle (ﷺ) to this effect but Allah's Apostle (ﷺ) did not disapprove of it.
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ الْعَنْبَرِيُّ، 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 أَتَحْلِفُ بِاللَّهِ قَالَ إِنِّي سَمِعْتُ عُمَرَ يَحْلِفُ عَلَى ذَلِكَ عِنْدَ النَّبِيِّ صلى الله عليه وسلم فَلَمْ يُنْكِرْهُ النَّبِيُّ صلى الله عليه وسلم .
Muslim
The Book of Tribulations and Portents of the Last Hour • Hadith 117