We don’t know. Hell, we can’t even narrow it down to a specific place with certainty. There is strong evidence in human settlements for use of fire anywhere from a few hundred thousand to 1 million years ago. When, exactly, is hard to ascertain; for instance, some sites which are claimed to hold the oldest evidence have been criticized as resembling the aftermath of wildfires.

It is also depends on what you mean by “discovered”: Early proto-hominids were almost certainly aware of fire and the concept of burning, so are we counting from when they realized “hey, I can take a burning thing and put it where I want it, and it will spread burning there?” Or are we only counting from when fire began to be used as a tool (e.g., for clearing brush or cooking)? Or when humans discovered how to start fires in the absence of a natural source?