My husband and I just bought a home. It has an old wall oven. As I was cleaning it today, I realized there is a hole about 1 inch across directly in the middle of the back wall. When I shine a flashlight in, I can see what I assume is a white wall behind it and what looks like insulation bits around the hole. What goes there? Is there any fireproof substance I can fill the hole with so it's safe? Please give me some ideas, thank you.

Cover the hole with sheetmetal.