What If...? (1977) #2

- First referance to a Professor or Smart version of the Hulk.

"What If the Hulk Had Always Had Bruce Banner's Brain?" – Bruce Banner, Reed Richards and Charles Xavier constructed an improved version of Cerebro called the Psychotron which merged their physical and mental beings into one being, X-Man. They were forced to use it (as it only worked once before blowing up), to defend the world from Galactus. They won the battle, at the expense of all of their super powers, and inadvertently turning Thing into a mindless monster, much like the Hulk.

SOURCE: marvel.wikia.com