Sahih Muslim Book 41, Hadith Number 6999.
Chapter : Story of Ibn Sayyid.
‘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 (may peace be upon him) to this effect but Allah’s Apostle (may peace be upon him) did not disapprove of it.