The background from phases 10 and above from Mario Bros. on the GBA (Included in the Mario Advance series and Mario and Luigi: Superstar Saga), as well as being used in the bonus stages. It even has the animated snow graphics falling in the background!
Since there was no available rip of it, I decided to rip it myself. Since the original background just used 8x8 tiles instead of 16x16 Map16 tiles, I had to slightly change the background around, although besides that, it should be almost exactly like the one featured in the GBA versions of Mario Bros!
No credit is necessary for me (Cheeyev), although it would be appreciated.