All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The story of Mukhayreeq was mentioned by the biographers, that he was a Jew from Madeenah who became a Muslim.
Ibn Hajar wrote a biography about him in al-Isaabah and he said:
“Mukhayreeq was a Jew from Nadhar (i.e. from Beni Nadheer) and Al-Waaqidi mentioned that he embraced Islam and was martyred in the Battle of Uhud.
Al-Waaqidi and al-Baladhri said: “It was said that he was from Beni Qaynuqaa’, and it was also said that he was from Beni Qatyoon. He was a scholar and he made a will of his wealth for the Prophet . His wealth was seven gardens: al-Maythab, as-Saa'ifah, ad-Dalaal, Husna, Barqa, al-A’waaf, Mashrabah um Ibraheem. So the Prophet assigned these gardens as endowment.” [End of quote]
It was the first endowment in Islam. Ibn Hajar said: “‘Amr ibn Sa’d ibn Mu'adh said: We asked about the first endowment in Islam, and it was said: with regard to the Muhajiroon: it was the charity of ‘Umar; as regards the Ansaar: it was the charity of the Prophet which was donated to him by Mukhayreeq, and then he (the Prophet ), assigned it as endowment.” [End of quote]
If it is confirmed that he embraced Islam, then it is believed that he made the donation while seeking the pleasure of Allah, The Almighty.
The reason for which he fought with the Prophet in the Battle of Uhud was quoted by Ibn Is'haaq in al-Maghaazi and it was also reported by others, such as Ibn Katheer and others.
Ibn Is'haac said: “Mukhayreeq was among those who were killed on the day of the Battle of Uhud. He was from the tribe of Beni Tha’labah ibn al-Fatyoon. On the day of the Battle of Uhud, Mukhayreeq said: “O Jews, you know that the victory of Muhammad over you is the truth. They said: “Today is a Saturday [meaning that they will be victorious]. He said: “There is no Saturday for you.” Then he took his sword and his belongings and said: “If I am killed then my wealth is for Muhammad and he may dispose of it as he wishes.” Then he went and fought alongside with the Prophet until he was killed. Thereupon, it was reported to us that the Prophet said: Mukhayreeq is the best of the Jews.” [End of quote]
Dr. Muhammed ibn 'Abdullah al-‘Awshan said in his book entitled: “Ma Shaa’a Wa lam Yathbut Min as-Seerah an-Nabawiyyah (What was spread as a rumor and was not proven from the Prophet's biography); this is how he quoted it without any chain of narrators. Also ibn Sa’d reported it from al-Waaqidi, and it is abandoned (i.e. Dha’eef –weak). Ibn Hajar attributed it in al-Isaabah to Umar Ibn Shabbah from az-Zuhri, as a Mursal Hadeeth (i.e. a report narrated on the authority of some of the Taabi'oon [the generation that followed the Companions]), which is one of the categories of weak Ahadeeth. Its chain of narrators includes Abdul-‘Azeez ibn ‘Imraan, whose narrations are considered weak according to most scholars. It was also narrated by az-Zubayr ibn Bakkaar from Muhammad ibn al-Hassan ibn Zabaalah, whose narration is weak as well. Ibn Hajar said: They considered him a liar.”
After research, we have not come across any quotation of the scholars of Hadeeth on the story that Mukhayreeq became a Muslim, and nothing was quoted neither about him making a will of his wealth, nor the Prophet praising him by saying that he was the best of the Jews. It is known that the authors of biography are very lenient in reporting the news.
Allah knows best.