Assalaamu alaykum. I have a question regarding doing business based on funding that is mixed from permissible and prohibited sources. I have several scenarios:
- The first scenario is the following: if I have a bike which I got from someone who stole it. I know that this is wrong, but let us say that I still use the bike for work. Does this makes my salary prohibited for me to use?
- Let us say that I have a business, like a minimarket, but the source of funding is from a conventional, interest-based bank. Is my income from the minimarket haram because of that?
What I am trying to ask is whether, basically, according to Islam, the fact that we got money from a prohibited source makes the income from anything of it haram or not. Please give me proof for this.Thank you for your attention. I am sorry for my English. May Allah reward you much.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His lave and Messenger.

The thief does not legally own the stolen item that he has in his hand, and accordingly he is not entitled to dispose of it lawfully by means of selling, offering it as a gift, or otherwise. The one who takes it from him bears a sin for accepting this item while knowing that it was stolen. He is liable for the due compensation (to the original owner) if he took possession of the stolen item.

In his Fataawa, ‘Ulaysh wrote, "Engaging in financial transactions with those who have acquired unlawful wealth: ... if the unlawful item of wealth is present in the possession of the thief or usurper or the like, it is prohibited to buy it from him or accept it as a price for something. It is prohibited to eat it if it is food, wear it if it is clothing, or accept it as a gift or loan. Whoever does any of these acts is as guilty as the thief or usurper in terms of consuming unlawful wealth, and he is obliged to clear himself of his liability from it."

However, using this bike and riding it to work does not render his salary unlawful. He is liable, though, to pay the fees of using it to its original owner.

The same applies to the person who takes a loan from an interest-based bank and uses this money for a lawful purpose; it is not prohibited for him to use this money or what he bought with it. The prohibition of such ill-gotten wealth is related to the person’s liability, and not to the ill-gotten wealth itself. For more benefit, please refer to fatwas 102369 and 130992.

We advise you to stop making such assumptions, because they open the door to Waswaas (obsessive whispers); verily, this is a serious illness that may ruin the person's religion and life.

Allah knows best.