The story that you mention was narrated in a well known hadeeth which is known as the hadeeth of al-Jassaasah. It is an important hadeeth which was one of the signs of his Prophethood, so we will give you the opportunity to benefit from it and enjoy reading it.
It was narrated that Faatimah bint Qays (may Allaah be pleased with her) said:
I heard the voice of the caller, the caller of the Messenger of Allaah (peace and blessings of Allaah be upon him), saying: Al-salaatu jaami’ah (prayer is about to begin), so I went out to the mosque and I prayed with the Messenger of Allaah (peace and blessings of Allaah be upon him). I was in the women’s row that was closest to the people. When the Messenger of Allaah (peace and blessings of Allaah be upon him) had finished his prayer, he sat on the minbar and he was smiling. He said: “Let each person stay in the place where he just prayed.” Then he said: “Do you know why I called you together?”
They said: Allaah and His Messenger know best. He said:
He said: “By Allah, I did not call you together for an exhortation or for a warning. I have called you together because Tameem al-Daari was a Christian and he came and swore allegiance and became Muslim, and told me something which agrees with what I was telling you about the Dajjaal (false messiah). He told me that he sailed in a ship with thirty men of Lakhm and Judhaam and they were tossed by the waves of the sea for a month. Then they came to an island at sunset. They sat in a small rowing-boat and landed on that island. They were met by a beast with a great deal of hair and they could not distinguish his face from his back because he was so hairy. They said: ‘Woe to you, what are you?’ It said: ‘I am al-Jassaasah.’ They said: ‘What is al-Jassaasah?’ It said: ‘O people, go to this man in the monastery for he keen to know about you.’ He (the narrator) said: When it named a man for us we were afraid of it lest it be a devil. Then we set off, rushing, until we came to that monastery, where we found the hugest man we had ever seen, bound strongly in chains with his hands tied to his neck and his legs bound from the knees to the ankles with iron shackles. We said: ‘Woe to you, who are you?’ He said: ‘You will soon find out about me; tell me who you are.’ They said: ‘We are people from Arabia who embarked on a ship, but the sea became wild and the waves tossed us about for one month, then they brought us to this island of yours. We took to the rowing-boats and landed on this island. We were met by a beast with a great deal of hair and we could not tell his front from his back because he was so hairy. We said: Woe to you, what are you? It said: I am al-Jassaasah. We said: What is al-Jassaasah? It said: Go to this man in the monastery for he is keen to know about you. So we came rushing to you and we fled from it because we could not be sure that it was not a devil.’ He (that chained person) said: ‘Tell me about the date-palm trees of Baysaan.’ We said: ‘What do you want to know about them?’ He said: ‘I am asking you whether these trees bear fruit.’ We said: ‘Yes.’ He said: ‘Soon they will not bear fruit.’ He said: ‘Tell me about the lake of Tabariyyah’ We said: ‘What do you want to know about it?’ He said: ‘Is there water in it?’ They said: ‘There is a great deal of water in it.’ He said: ‘Soon it will dry up.’ Then he said: ‘Tell me about the spring of Zughar (which is in the south of Syria).’ They said: ‘What do you want to know about it?’ He said: ‘Is there water in the spring and do the people grow crops with the water of the spring?’ We said to him: ‘Yes, there is plenty of water in it and the people grow crops with its water.’ He said: ‘Tell me about the Prophet if the unlettered; what has he done?’ We said: ‘He has left Makkah and has settled in Yathrib (Madeenah).’ He said: ‘Do the Arabs fight against him?’ We said: ‘Yes.’ He said: ‘How did he deal with them?’ We told him that he had prevailed over the Arabs in his vicinity and they had shown obedience to him. He said to us: ‘Has it really happened?’ We said: ‘Yes.’ He said: ‘If it is so that is better for them that they show obedience to him. Now I will tell you about myself. I am the Dajjaal and soon I will be given permission to emerge. So I will come out and travel in the land, and will not spare any town but I will stay for forty nights, except Makkah and Taybah (Madeenah). They are both forbidden to me; every time I try to enter one of them, I will be met by an angel with a sword in his hand, who will bar my way, and on every route there will be angels guarding it.’ She said: Then the Messenger of Allaah (peace and blessings of Allaah be upon him) struck the minbar with his staff and said: “This is Taybah, this is Taybah, this is Taybah,” meaning Madeenah. “Did I net tell you this before?” The people said: Yes. [The Prophet (peace and blessings of Allaah be upon him) said:] “I liked the story of Tameem because it agrees with what I used to tell you about him and about Makkah and Madeenah. But he is in the Syrian Sea (Mediterranean) or the Yemeni Sea (Arabian Sea). No, rather he is in the east, he in the east, he is in the east,” and he pointed towards the east with his hand. She said: I memorized this from the Messenger of Allaah (peace and blessings of Allaah be upon him).
Narrated by Muslim in his Saheeh (2942), so it is a saheeh hadeeth. It was also narrated by the scholars in their books, with their isnaads from Faatimah bint Qays (may Allaah be pleased with her). Al-Tirmidhi (may Allaah have mercy on him) said in al-Jaami’ al-Saheeh (2253): This is a saheeh ghareeb hadeeth. End quote. Ibn ‘Abd al-Barr said in al-Istidhkaar (7/338): It is saheeh in its isnaad and transmission. End quote.
And Allaah knows best.