Narrated Muhammad bin Al-Munkadir:I saw Jabir bin `Abdullah swearing by Allah that Ibn Sayyad was the Dajjal. I said to Jabir, "How can you swear by Allah?" Jabir said, "I have heard `Umar swearing by Allah regarding this matter in the presence of the Prophet (ﷺ) and the Prophet (ﷺ) did not disapprove of it

English translation · Sahih al-Bukhari