What is the ruling on dealing with non-Islamic banks?

Usury is a grave sin for which Allah has declared war on its perpetrator, as have His Messenger. The Prophet, peace and blessings be upon him, cursed the one who consumes usury, the one who pays it, and its two witnesses. Therefore, one must avoid dealing with usurious banks except in cases of extreme necessity, such as fearing for one's money during travel with no other safe place available, provided that the money is withdrawn immediately once the necessity ceases. This is because necessity is judged by its measure, and because keeping the money there helps them in sin.