Answer
Wa `alaykum as-Salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Mosques are related to devotional affairs like worship, so they should be financed through lawful (halal) ways. Hence, it is not permissible for you to pay the interest of your money to the mosque.
Responding to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:
Earned interest (according to the Shari`ah it is not really earned) should be given to the poor and needy and the like of objectives that serve the community at large.
I would not suggest giving it for operating expenses of the mosque unless it is specifically used for the bathroom areas and their like, not for anything related to the prayers area because it is more respected than being financed from haram.
Allah Almighty knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.