For those who have heard Testify by Stevie Ray Vaughan on Texas Flood, who originally did the song?

Thanks In advance
As far as I know the original artist was unknown. I don't think it's an official cover of anyone... he probably came up with it during a jam. Don't take my word for it, though...
Looking for my India/Django.
Parliament did a song called 'Testify'. Could be them, but it seems unlikely
How's the Larry Davis Texas Flood version?
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the davis texas flood is good, but its not that great, its worth a listen though, especially if you're going to play it to see where its coming from