Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 3199
Abu Huraira and Sa'd reported Allah's Messenger (way peace be upon him) as saying: O Allah, bless the people of Medina in their mudd, the rest of the hadith being the same, and in It (this is also mentioned):" He wo intends to do harm to its people, Allah would efface him just as salt it dissolved in water."