What does the Quran say about interest (riba)?
Allah says:
"Those who consume interest will stand on the Day of Judgement only as one stands who has been struck by Shaytan into insanity. That is because they say, 'Trade is just like interest.' But Allah has permitted trade and forbidden interest." - Surah Al-Baqarah (2:275)
And more strongly:
"O you who believe, fear Allah and give up what remains of interest, if you are believers. And if you do not, then be informed of a war from Allah and His Messenger." - Surah Al-Baqarah (2:278-279)