Wa`alaykumas-salam 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.
Dear sister in Islam, thank you for your question, which is a sign of your good faith and keenness to have a lawful earning. May Allah bless you and grant you halal (Arabic for: permitted according to Islamic teachings) and blessed sustenance.
As for your question, it is permissible to take this inheritance even if your mother had deposited it in a usury-based bank. However, your duty is to give out any sum of money taken as interest in charity and to be keen to distribute this inheritance according to Shari`ah.
In his response to your question, Sheikh Faysal Mawlawi, deputy chairman of the European Council for Fatwa and Research, stated,
It is lawful to take the inheritance, Allah willing, as your father denied that his money is unlawful and you have no evidence to prove that it came from unlawful activities.
Thus, the sum of money that your mother received as a gift from your father and deposited in a bank is lawful even if she deposited it in a usury-based bank. The sin of depositing money in a usury-based bank does not make the money unlawful.
However, the interest of this sum of money is considered unlawful, and it should be spent on the poor and needy. Thus, the inherited money, after excluding the interest, is lawful and you should pay zakah (Arabic for: alms obligatory according to Islamic teachings) for it when you receive it.
Excerpted with slight modification from iums.com.
