From Sahih Muslim (Hadith 3363):
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
Reference: Muslim 3363