That was a part of a crossover between "The Incredible Hulk" and "Web of Spider-Man" series. I think it was in the "Web" issue. Spidey was struggling to prevent a billboard from falling and injuring people. The Hulk (who the world currently thought was dead) made the threat against Spider-Man to bust his chops. Spidey promised he wouldn't tell anyone that he (the Hulk) was alive if the Hulk helped him with the billboard. That satisfied the Hulk and he easily hefted it away to safety.
*I just did a check of the Marvel chronology because I wanted to check my memory of this and I had everything correct except it was in the Incredible Hulk #349. Not too shabby for something I read over 20 years ago!*