Ruffalo took to Twitter to reveal the following Thor: Ragnarok set video where we see the actor with giant Hulk hands.
Ruffalo runs at the camera making a Hulk noise and does a Hulk punch!
Updated: with second set video below.
While the plot hasn't been officially released, it's probably a safe bet that at some point in the movie we'll see Thor battle the Hulk prior to the pair teaming up to take down Hela and her minions (and maybe Loki?).
"Thor: Ragnarok" has a November 3, 2017 release also starring Chris Hemsworth as Thor, Jeff Goldblum as Grandmaster, Cate Blanchett as Hela, Anthony Hopkins as Odin, Karl Urban as Skurge, and Tessa Thompson as Valkyrie.