Everything in that picture, except for the fence (placeholder) and the mountains is 100% original. The clouds, water, rock, grass, tree and even the Sonic sprite used are all brand new and not from any other game (or at least I hope not, as it'd mean someone has used the art without permission). You may recognise the foreground from the screenshots topic on SSRG, as SpirituInsanum has shown it off there before, but that's the only place you should have seen it.
Still, it doesn't look like a Sonic level unless you're planning to make ALL of the assets from scratch (even the sprites themselves). Both the Hedgehog and the Bridge looked out of place compared to the rest of the level.