The Jews of Arabia spoke Arabic although their dialect was interspersed with Hebrew for they had not completely given up their religious aspirations. In regard to the missionary activities of the Jews, Dr. Israel Welphenson says:
“There is less uncertainty about the opportunities offered to the Jews in consolidating their religious supremacy over Arabia. Had they so willed, they could have used their influence to the best advantage. But as it is too prominent among every student of Jewish history, they have never made any effort to invite other nations to embrace their faith, rather, for certain reasons, they have been forbidden to preach this to others.” (Dr. Israel Welphenson; Al-Yahud fi Balad il-‘Arab, p. 72)
Be that as it may, many of the Aus and the Khazraj and certain other Arab tribes had been Judaized owing to their close social connections with the Jews or ties of blood. Thus, there were Jews in Arabia, who were of Israelite descent, with a fraction of Arab converts. The well-known poet K’ab b. Ashraf (often called an an-Nadir) belonged to the tribe of Tayy. His father had married in the tribe of Bani an-Nadir but he grew up to be a zealous Jew. Ibn Hisham writes about him: K’ab b. Ashraf who was one of the Tayy of the sub-section of Bani Nabhan whose mother was from the Bani al-Nadir. (Ibn Hisham, Vol. P. 514).
There was a custom among the pagan Arabs that if the sons of anybody died in infancy, he used to declare to God that if his next son remained alive, he would entrust him to a Jew to rear him up on his own religion. A tradition referring to this custom finds place in the Sunan Abu Dawud.
“Ibn ‘Abbaas said: Any woman whose children died used to take the vow that if her next child remained alive, she would make him a Jew. Accordingly, when Banu an-Nadir were deported they had the sons of Ansaar with them; they said, “ We would not forsake our sons”, thereupon the revelation came: “There is no compulsion in religion.” (Sunan Abu Dawud, Kitaab-ul-Jihad, Vol. II)
AUS AND KHAZRAJ
The two great Arab tribes of Madinah, Aus and Khazraj, traced a common descent from the tribe of Azd belonging to Yemen from where successive waves of emigrants inundated the northern regions from time to time. The exodus was brought about by a variety of reasons, some of which were the unstable political conditions in Yemen, Abyssinian aggression and disruption of irrigation system supporting agriculture after the destruction of Ma’rib dam. However, both the Aus and Khazraj came down to Madinah after the Jews. The Aus settled down in ‘Awali, an area in the south-east of Madinah while the Khazraj occupied the lands in the central and northern parts of it. With the northern part of the city being low-lying, nothing intervened between the abode of the Khazraj and Harrata Wabrah in the West. (Makkah wal Madinah, p. 311).
The Khazraj consisted of four clans: Malik, ‘Adiy, Mazin and Dinar, all co-laterals to Banu Najjaar and also known as Taym Al-Lat. Banu Najjaar took up residence in the central part of the city where the Prophet’s mosque now stands. The Aus, who have settled in the fertile, arable lands were the neighbors of the more influential and powerful Jewish tribe. The lands occupied by Khazraj were comparatively less fertile and they had only Banu Qaynuqaa as their neighbors. (Makkah wal Madinah, p. 311).
It is rather difficult to reckon the numerical strength of Aus and Khazraj with any amount of certainty, but an estimate can be formed from different battles in which they took part after the Prophet’s (Sallallahu ‘Alaihi Wa Sallam) departure to Madinah. The combatants drafted from these two tribes on the occasion of the conquest of Makkah numbered four thousand. (Al-Imta, Vol. I, p. 364)
When the Prophet (Sallallahu ‘Alaihi Wa Sallam) migrated to Madinah, the Arabs were powerful and in a position to play the first fiddle. The Jews being disunited had taken a subordinate position by seeking alliance either with the Aus or the Khazraj. Their mutual relationship was even worse for they were more tyrannical to their comrades in religion in times of clashes than to the Arabs themselves. It was due to the antipathy and bitterness between them that the Bani Qaynuqaa were forced to abandon their cultivated lands and resorted to working as artisans. (Makkah wal Madinah, p. 322)
The Aus and the Khazraj, too, often came to the scratch. The first of these encounters was the battle of Samyr while the last one, the battle of Bu’ath, was fought five years before the Hijrah.(6) The Jews always tried to sow dissension between the Aus and Khazraj and made them run afoul of one another so as to divert their attention from them. The Arab tribes were conscious of their nefarious activities – “the fox” was the popular nickname they had given to the Jews.
An incident related by Ibn Hisham, on the authority of Ibn Is’haq, sheds light upon the character of the Jews. Sh’ath b. Qays was a Jew, old and bitter against the Muslims. He passed by a place where a number of the Prophet’s companions from Aus and Khazraj were talking together. He was filled with rage seeing their amity and unity. So he asked the Jewish youth friendly with the Ansaars to join them and mention the battle of Bu’ath and the preceding battles, and to recite some of the poems concerning those events in order to stir up their tribal sentiments.
The cunning device of Sh’ath was not in vain, for later on the two tribes had been at daggers drawn in the past. Their passions were aroused and they started bragging and quarreling until they were about to unsheathe their swords when the Prophet (Sallallahu ‘Alaihi Wa Sallam) came with some of the Muhaajirun. He pacified them and appealed to their bonds of harmony brought about by Islam. Then the Ansaars realized that the enemy had duped them. The Aus and Khazraj wept, embraced and welcomed back one another as if nothing had happened. (Ibn Hisham, Vol. I, pp. 555-6)
 Fath-ul-Bari, Vol. VII, p. 85. See Ibn Kathir for the detailed account of the battle of Bu’ath.