K'ab Al-Ahbar. Ka'b. Ka'ab.
He converted to Islam.
But he was originally Jewish.
Yakub ben Shallum was a scholar, and he was educated in Pumbedita. He continued to follow Rabbinic tradition such that later Islamic historians questioned whether he ever "converted" to Islam. Yaakov was known as Ka'b Ibn Mati' Al-Himyari, also nicknamed Abu Ishaq, from the clan of Dhu Ru'ayn came to Medina during the time of 'Umar. He was a prominent Rabbi and became known as Ka'b Al-Ahbar. He declared his Islam and resided in Medina during 'Umar's reign. He stayed in Medina until the days of 'Uthman.
He was a Jew, a native of Yemen. His complete name was Abu Isḥaḳ Ka'b b. Mati' b. Haisu (Hosea ?). On account of his theological learning he was styled "Al-Ḥibr" or "Al-Aḥbar," which is an adaptation of the Hebrew "ḥaber." He brought much of the Isrāʾīliyāt (things Jewish) into Islamic tradition. He was said to have declared: "I would rather fornicate thrity-three times than consume a dirham of usury while Allâh knows that I knowingly consumed it as usury". He accompanied the Caliph Umar ibn al-Khattab during his conquest of Al Quds (Jerusalem) in the year 638 AD and was the Caliph Umar ibn al-Khattab's counselor and advisor on the holy sites of Al Quds (Jerusalem).
But his fame is related to the Temple Mount:
...Ka'b was with Khalif 'Umar when he entered Jerusalem...In 642CE...'Umar he summoned Yaakov (Ka’b al-Ahbar) to find his thoughts on the matter...[of] "Where do you think better to build the musalla, oratory? " Ka’b replied: "Beside the rock". In another narration: "If you seek my advice, I prefer to pray behind the rock, i.e. the rock would be the qiblah."
'Umar said: "O Jewish person [son of a Jewish woman], you are mixing your Jewishness [with Islam]. I shall build it at the forefront of the mosque, as to us belong the forefronts of the mosques, and I saw you and the way you took off your shoes." He said: "I liked to conduct [be in contact with] it with my foot."
'Umar observed that Ka'b took off his shoes. This was not in accordance with Rabbinic custom [Zohar, Parshas Balak], rather Islamic – specifically Khawarji – custom...He also appeared that he was requesting that the qiblah be changed back to Jerusalem.
When 'Umar started to make repairs on the Bayt al-Maqdis, Temple Mount [!], from the trash which the Romans buried it in, 'Umar heard the takbir, God is Greater, announced from behind him. 'Umar was averse to evil do anything of showy false piety. He said: What is this? They replied: Ka’b said God is Greater and people repeated after him. 'Umar said: Bring him here. Ka’b said: O Amir al-Mu’minin, a prophet foretold of what you did today five hundred years ago. He asked: How is that? He replied: The Romans attacked Banu Israel and buried it (Bayt al-Maqdis), till you were appointed a ruler, when Allah delegated a prophet over the sweepings who said: O Jerusalem, I have good tidings for you, al-Faruq, has come to cleanse you of that which inside you...
This Haddith seems to show Yaakov's true intentions. Perhaps with the liberation of Jerusalem by the Muslims, Yaakov began to be interested for the first time in mixing Judaism and Islam to push Muslims to achieve "Jewish goals", as it says "take vengeance .. upon the Romans". This is exactly what happened in 614 CE during the Persian conquest of Jerusalem. The Jews & Persians took vengeance on the Romans and it is said that several tens of thousands of Christians died. This turned the war against the Persians and brought about the fall of the Persian empire. So it obviously would have been unwise for 'Umar to take up this cause. 'Umar ordered Jews removed from Masjid al-Aqsa to a synagogue outside the Temple Mount, outside its south western corner...
...Islamic histories mention that around this time Abu Dharr, a strict ascetic, became incensed upon hearing Ka'b giving verdicts in Islamic law. Yaakov had justified 'Uthman's withholding State funds from being distributed to the poor. Abu Dharr struck and perhaps killed Yaakov with his rod, saying "Son of a Jewish lady, are you trying to teach us our religion?" (His age at death is recorded as "exceeding 100 years of age").
So, there is a Temple Mount according to Islamic history. And Jews prayed therein (or adjacent). And Muslims did ban Jews from doing so. And Muslims never forgave his Jewish origin.