Sahih Bukhari Volume 004, Book 055, Hadith Number 565.
Narated By Zainab bint Jahsh : That the Prophet once came to her in a state of fear and said, "None has the right to be worshipped but Allah. Woe unto the Arabs from a danger that has come near. An opening has been made in the wall of Gog and Magog like this," making a circle with his thumb and index finger. Zainab bint Jahsh said, "O Allah's Apostle! Shall we be destroyed even though there are pious persons among us?" He said, "Yes, when the evil person will increase."