His name was Ka‘ab al-Aḥbār, a Rabbi from Yemen who became a convert to Islam.
From the Jewish Encyclopedia:
On account of his theological learning he was styled "Al-Ḥibr" or "Al-Aḥbar," which is an adaptation of the Hebrew "ḥaber." He lectured on the Koran and the career of Mohammed, not from the merely exegetical and biographical points of view, but in a homiletic and haggadic manner, just as Abdallah b. Salam had done. Both these men laid the foundation for the legends which glorify Mohammed's youth and prophetic call.
He was an influential figure in the reigns of the Khalifs Umar and Uthman. Associated with the development of the Sunni tradition...
It is reported that when Umar marched into Jerusalem with an army, he asked Ka‘ab: "Where do you advise me to build a place of worship?" Ka‘ab indicated the Temple Rock, now a gigantic heap of ruins from the temple of Jupiter. The Jews, Ka‘ab explained, had briefly won back their old capital a quarter of a century before (when Persians overran Syria and Palestine), but they had not had time to clear the site of the Temple, for the Rums (Byzantines) had recaptured the city. It was then that Umar ordered the rubbish on the Ṣakhra (rock) to be removed by the Nabataeans, and after three showers of heavy rain had cleansed the Rock, he instituted prayers there. Umar is said to have fenced it and, some years later, an Umayyad Khalif built the Dome of the Rock over the site as an integral part of the Aqsa Mosque. Until this day, the place is known as ḳubbat al-ṣakhra, the Dome of the Rock.
According to tradition, Ka‘ab believed that "Every event that has taken place or will take place on any foot of the earth, is written in the Tourat (Torah), which God revealed to his Prophet Moses". He is said to have predicted the death of Umar using the Torah. According to one narration, Ka‘ab told Umar "you ought to write your will because you will die in three days." Umar responded "I do not feel any pain or sickness". Abu Lulu assassinated Umar two days later.
After Umar's death Ka‘ab went to Syria and became one of Mu‘awiyah's advisers. He died in Hims during the Caliphate of Uthman, said to have been over 100 years of age.
Sunni Islam believes his reliability whereas Shia denounce him has a Jewish agent. Shia Islam not able to comprehend mystical people who can know the future through merkerba ascension or through knowledge of mystical letters in the Torah have unfairly denounced him as a heretic. Sunni Islam condemns Shia for being started by a Jew in an ironic twist. Yet none of his reports are mentioned in the hadith except one.
Within the Sunni tradition Ka'ab is seen as a trustworthy scholar. Ibn Hajar Asqalani, a 14th-century Sunni Shafi'i scholar, wrote,
Ka`b Ibn Mati` al-Himyari, Abu Ishaq, known as Ka`b al-Ahbar, is trustworthy (thiqah). He belongs to the 2nd [tabaqah]. He lived during both Jahiliyyah and Islam. He lived in Yemen before he moved to Sham [~Syria]. He died during the Caliphate of `Uthman exceeding 100 years of age. None of his reports are in al-Bukhari. He has one narration in Muslim from Abu Huraira from him on the authority of al-A`mash from Abu Salih.
Related article (criticism of him): http://www.al-islam.org/shiite-encyclopedia-ahlul-bayt-dilp-team/companions-and-jewish-influence-part-1