Abu Dawud Book 32, Hadith Number 4317.

Narated By Jabir ibn Abdullah : 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 Apostle of Allah (PBUH), but the Apostle of Allah (PBUH) did not make any objection to it.