How awesome is this song? Listen.
The drum part was written by legendary guitarist Jeff Beck, but it was performed by Wonder in the studio. Stevie originally offered the song to Beck, but ended up recording it first. Wonder also plays the clavinet which is the sharp funky riff that sort of sounds like a cross between an electric guitar and a clavichord. He is also responsible for the bass, which is also coming out of a Moog synthesizer. The trumpets and saxes are not him, he let someone else record those, but I would not be surprised if he wrote their parts out.
There's not much I can add here. The song is a perfect picture of funk. My favorite performance of it is the one he did on Sesame Street in 1973. It has been covered by hundreds of artists. You should check out the one by Beck, Bogert & Appice if only because it was going to be the original recording until Berry Gordy told Wonder that he better record it first. It's still funky, but it's more progressive rock. Wonder's version is perfect, and very apropos considering the date.