My favorite 'Sherman mouthing off' moment is when he mouthed off to gigantic Trent Williams and was then punched in the face. Sherman barely moved and started laughing. I'm thinking if Brady punched him it would have turned out differently.
Found it:
Williams punches Richard Sherman
My understanding is that he didn't actually say "You mad bro?" Someone tweeted the infamous pic of him and Brady, along with that quote, and Sherman then adapted it.