Through another chain of narrators it is mentioned in Al-Jaraih that when the Messenger of Allah (S) was staying at Quba and had said that till Ali does not join him, he will not proceed to Medina, when Salman used to ask the people about the Prophet at that time he had been enslaved to a Jew of Medina and he used to work in his date orchard.
When Salman came to know that Prophet was staying in Medina, he took a tray of dates and came to him and said: I have heard that you are poor people staying at this place, so I brought these dates as my Sadaqah, please eat from them. The Holy Prophet (S) told his companions to eat from it with the name of Allah, but he did not eat from it himself.
Salman watched all this with interest and remarked in Persian that it was the first sign. Then he filled the tray with dates and told the Prophet: I noticed that you did not partake from the dates of Sadaqah, so I have brought these dates for you as gift. The Prophet partook from them and also told his companions to follow suit.
After that Salman picked up the tray and remarked: This is the second sign. And he went to the back of the Prophet and noticed the seal of prophethood. And pleaded to the Prophet that he was enslaved to a Jew, so what he advises him? He said: Go and make a deal with him to free you at a cost, so that I can arrange it and have you freed. Salman came to his Jew master and said: I have become a Muslim and have started following the Prophet who has arrived in this town. So you will not be able to gain anything from me.
Please take an amount of cash and free me. He said that he would free him only if Salman planted 500 date trees for him. And that he would serve him only till they bear fruit; and also to give him 40 Awqiya of gold and each Awqiya is equal to 40 Mithqal. Salman reported this to the Prophet who said: Go and accept his conditions. Salman went there and made the agreement.
The Jew was under the impression that this will take a few years. Thus Salman remained with the Prophet according to that agreement. The Prophet said: Go and bring 500 seeds of date. I presented them. The Holy Prophet (S) said: Give them to Amirul Momineen (a.s.) and show me the place where the man wants the trees planted. The Prophet came to that place with Ali (a.s.) and Salman.
He made holes in the ground with his finger and told Ali (a.s.) to put a seed in each. Then he cast some earth on it and when he separated his fingers, water used to gush out of them. The same procedure was carried out with the next seed. When he finished with the second seed, the first used to become a tree. And when he sowed the third, the second grew into a tree and the first bore fruits.
In this way all the seeds were sowed and 500 trees grew up and fructified. When the Jew witnessed this strange phenomenon, he said: The Quraish are right that Muhammad is a magician (Refuge of Allah). He said: Now bring the gold. The Prophet picked up a rock which turned to gold by his miracle. The Jew said: I have not seen gold better than this. Then he weighed it and it was exactly 40 Awqiya, neither less nor more. Salman says that I returned with the Prophet a freed man and I began to live with him.