In Cycle render engine you have an option to disable direct visibility of the objects to the camera. 

Open _Object Data tab_ and in _Ray Visiility_ section disable this option on all the objects that you want to be invisible in render.

![enter image description here][2]

You can do the same with the background in _World_ tab.

![enter image description here][1]




  [1]: https://i.sstatic.net/3JGBi.png
  [2]: https://i.sstatic.net/aWk0N.png