What is the ruling of Islam on guaranteeing a brother in a bank loan?

If the loan is interest-based (usurious), it is not permissible for you to guarantee the borrower, because doing so constitutes assisting in prohibited usury. The Permanent Committee has stipulated that cooperation in such matters is not permissible, based on Allah’s saying: (And do not cooperate in sin and aggression), and based on the hadith: "Allah has cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses," and he said: "They are all equal."