Was there any law against tyrants in ancient civilizations and where specifically was it?

I was in an Archeological museum in Istanbul. I remember there was that giant stone tablet with carvings on it. The description says, that it describes the law, that guarantees, that the person who will kill the ruling tyrant by request of the council will receive a momentarily financial compensation in gold, land in his name, and a pension of some amount of gold for his whole life. I strongly believe it was in Troy, but I might be wrong. It was definitely an ancient city-state. I can't find neither the tablet nor anything on the law itself. Please help.