Backgrounds from Green Hill Zone.
Sonic 1 backgrounds :
Backgrounds from Green Hill Zone.
The back of the scene from Green Hill, then two top of the scene, isolated from Green Hill.
Backgrounds and top-of-scenes (or isolated layers) from Marble Zone.
In bonus, elements to add on some secnes :
Backgrounds from Spring Yard Zone.
Backgrounds from Labyrinth Zone.
Backgrounds from Star Light Zone.
Backgrounds from Scrap Brain Zone.