Imam Muhammad Baqir (a.s.) has said that the ‘Islam’ of this Jew did not do any good and he acted in bad ways thereafter also.
When they dispersed from the presence of the Holy Prophet (S) they said to one another secretly that Muhammad is not a true prophet. When Kaab bin Ashraf wished to ride his donkey, the animal reared up and threw him down, injuring him badly. He rose up again and mounted, but the animal once again threw him down.
This happened again and again. At the seventh or eighth attempt, the animal spoke up by God’s command: O Servant of God! You are a very bad fellow. You did not believe even after observing His signs. You remained a disbeliever. I am only an animal.
But Allah has honored me with belief in His Oneness and I give witness that there is no God except Allah, Who is the Creator of entire universe and is Mighty and Powerful and that Muhammad (S) is His servant and Messenger and the chief of all men of Paradise and who has been raised by Allah so that he may make those people successful and fortunate whose being so (faithful) is in the knowledge of Allah and so that He may make them stonehearted and unfortunate whose being so (faithless) is also written in the Knowledge of Allah and I give witness that Ali Ibn Abi Talib (a.s.) is one who, if he makes one fortunate, Allah also makes that one fortunate and will enable him to accept admonition and to learn manners and to abide by His orders and to refrain from what has been made unlawful by Him, because Almighty Allah will, through his sword and revengeful attacks, disgrace the enemies of Muhammad (S) until they either believe in him, otherwise remain in revolt and finally send them to Hell by his sword.
Now it is not becoming for any disbeliever to ride on me. Only he can ride me who believes in Only One Allah and testifies to all sayings of Muhammad (S) and regards all his deeds correct, especially of his cousin and legatee, Ali (a.s.), who is his vicegerent and heir of all of his knowledges, defender of his religion, protector of his community, repayer of his debts, fulfiller of his promises, friend of his friends and the foe of his foes and who believes that Muhammad (S) is totally right in appointing him (Ali) as his deputy and thereby abides by his orders.
Then the Holy Prophet (S) told Kaab bin Ashraf: O Kaab! Your donkey is better than you, because it does not allow you to ride it. So now you may sell it to any of our believer brother. Kaab said: I also don’t need it now, because (God forbid) it is under your spell of magic. Hearing this, the donkey spoke up again: O enemy of Allah! Stop speaking ill of the Prophet. By God, had I not been afraid of the opposition of the Prophet I would have killed you, finishing you with my hooves and teeth. Kaab felt ashamed and could not say anything.
He became restless in heart but haughtiness overpowered him and he did not bring faith. Thabit bin Qays bought that ass for 100 dinars, rode it and came up to the Holy Prophet (S) very often when the animal behaved very mildly, nicely and lovefully. The Holy Prophet (S) told Thabit: O Thabit! This ass has become so nice and obedient due to your faithfulness.