It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said to the Muslims: "I have been shown the place of your emigration. I saw a salty land with date-palm groves lying between two black stony tracts (labatayn)." And "labatayn" means the two harrahs (tracts of black lava rock) of Madinah. This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it!