What is the ruling on disposing of a large sum of money that was transferred to my bank account, whose source is unknown, given that the bank confirmed the validity of the transfer and that there are no complaints concerning it?

A person is not permitted to dispose of something he does not own, and ownership is not established merely by money being deposited into a bank account. As long as you do not know the owner of the amount, and whether it was deposited into your account by mistake or as a gift, you are not permitted to dispose of it. You must check with the bank to ascertain the source of the money, and then check with the source, because the Prophet, peace and blessings be upon him, said: "The feet of the son of Adam will not move on the Day of Resurrection from his Lord until he is asked about five — and among them he mentioned — and his wealth, from where did he acquire it?"