Allah's Apostle said

Virtues of Madinah
Bukhari :: Book 3 :: Volume 30 :: Hadith 104
Narrated Abu Huraira:
Allah's Apostle said, "There are angels guarding the entrances (or roads) of Medina, neither plague nor Ad-Dajjal will be able to enter it."
Bukhari :: Book 3 :: Volume 30 :: Hadith 105
Narrated Anas bin Malik:
The Prophet said, "There will be no town which Ad-Dajjal will not enter except Mecca and Medina, and there will be no entrance (road) (of both Mecca and Medina) but the angels will be standing in rows guarding it against him, and then Medina will shake with its inhabitants thrice (i.e. three earth-quakes will take place) and Allah will expel all the nonbelievers and the hypocrites from it."
Bukhari :: Book 3 :: Volume 30 :: Hadith 106
Narrated Abu Said Al-Khudri:
Allah's Apostle told us a long narrative about Ad-Dajjal, and among the many things he mentioned, was his saying, "Ad-Dajjal will come and it will be forbidden for him to pass through the entrances of Medina. He will land in some of the salty barren areas (outside) Medina; on that day the best man or one of the best men will come up to him and say, 'I testify that you are the same Dajjal whose description was given to us by Allah's Apostle .' Ad-Dajjal will say to the people, 'If I kill this man and bring him back to life again, will you doubt my claim?' They will say, 'No.' Then Ad-Dajjal will kill that man and bring him back to life. That man will say, 'Now I know your reality better than before.' Ad-Dajjal will say, 'I want to kill him but I cannot.' "
Bukhari :: Book 3 :: Volume 30 :: Hadith 107
Narrated Jabir:
A bedouin came to the Prophet and gave a pledge of allegiance for embracing Islam. The next day he came with fever and said (to the Prophet ), "Please cancel my pledge (of embracing Islam and of emigrating to Medina)." The Prophet refused (that request) three times and said, "Medina is like a furnace, it expels out the impurities (bad persons) and selects the good ones and makes them perfect."