He had however adopted a new form of defense, unknown in Arabia at that time: Muslims had dug trenches wherever Medina lay open to cavalry attack. The idea is credited to a Persian convert to Islam, Salman. The siege of Medina began on 31 March 627 and lasted for two weeks.[34] Abu Sufyan's troops were unprepared for the fortifications they were confronted with, and after an ineffectual siege, the coalition decided to go home.[35] The Qur'an discusses this battle in verses 9-27 of sura 33, Al-Ahzab.[36][37]