All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
Perhaps the questioner is asking about Ibn Sayyaad? Many of the Companions and scholars were so confused regarding the identity of this man that they thought he was the Anti-Christ that would appear towards the end of time. This opinion was maintained by ‘Umar ibn Al-Khattaab, his son ‘Abdullaah, Abu Tharr and Jaabir . It was reported by Muhammad ibn Al-Munkadir that Jaabir ibn ‘Abdullaah used to vow that Ibn As-Sayyaad was the Dajjaal. It was also reported that ‘Umar used to swear by Allaah in the presence of the Prophet, sallallaahu ‘alayhi wa sallam, that Ibn Sayyad was himself the Anti-Christ, and the Prophet, sallallaahu ‘alayhi wa sallam, stayed silent about it. [Muslim]
However, the predominant opinion according to competent research scholars is that Ibn Sayyaad was a Dajjaal (charlatan) but he is not the Anti-Christ who will appear towards the end of the world. This view was maintained by Ibn Katheer and he supported it by quoting the Hadeeth that was narrated by Faatimah bint Qays Al-Fihriyyah . This Hadeeth is Saheeh (authentic) and narrated by Muslim in his book.
The Roman King Heraclius is the one who asked Abu Sufyaan about the characteristics of the Prophet, sallallaahu ‘alayhi wa sallam, his ancestors, and followers. This was mentioned in a long Hadeeth that was narrated by Ibn ‘Abbaas and his father. In that Hadeeth, Ibn ‘Abbaas and his father, said that Abu Sufyaan ibn Harb told him that Heraclius sent a messenger to him with a convoy from Quraysh and asked him the questions mentioned above. [Al-Bukhari]
Allaah Knows best.