All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
There is no harm on you in taking this money and benefiting from it. It is a provision that Allaah has blessed you with. The Prophet said: "If you are given money when you neither hoped for it nor asked for it, then take it, otherwise do not take it." [Al-Bukhari and Muslim]
Besides, you do not have to find out the source of this money, but if you know that it is a bank interest, or that it is not his own money, then it is not permissible for you to take it (or benefit from it).
Allaah knows best.