Is it permissible to eat the food of one who trades in qat?

Question:
He says: My father trades in qat. Is it permissible for me to eat at my father's house while visiting him?

Answer:
No. If you know for certain that this food is from the price of qat or tobacco, it is not permissible for you to eat from it, because it is forbidden. When Allah forbids something, He forbids its price, as in the hadith: If he has lawful food and forbidden food, and you do not know which is which, the default is permissibility; you eat with the intention that it is from the lawful.