bro , i found a story here ...may be , u r looking for it. I did not find the word martyr about him here .
The most remarkable and the least known of these non-Muslim helpers was a Jewish Rabbi who fought along with the Muslims and died in the battle of Uhud.
Ibn Hisham records his name as Mukhayriq and identifies his tribe as the Banu Tha'lah.
His tribesmen tried to dissuade Mukhayriq from joining the Muslims in this war by pointing out that as the battle was being fought on a Saturday, the day of Jewish Sabbath, he had a religious excuse not to participate.
Mukhayriq argued that the covenant Jews and taken with the Muslims took precedence over any religious and other excuses.
http://www.ispi-usa.org/muhammad/muhammad9.html
there was another Jew who later embraced Islam.
ABDULLAH IBN SALAAM [radhiallaahu anhu]
His original name was al-Husain ibn Salam. He was a learned rabbi, and one respected by all, for his righteousness and knowledge
......Abdullah ibn Salam dedicated himself to Islam with an eagerness that surpassed his zeal when he was a Jew.
He adored reciting the Qur'an, letting its meaning and feeling seep into his mind, his heart and the marrow of his bones. He was passionately attached to the Prophet, sallallahu alayhe wasallam, and sought always to be close to him learning from him his sayings and actions
http://www.alinaam.org.za/library/ibnsalaam.htm
I thought so I just didn't want to give the wrong answer:
