Stallone shared his early birthday wish, along with a photo of the Schwarzenegger in his Pumping Iron days side-by-side with himself as Rambo in First Blood.
"Pre - HAPPY BIRTHDAY ARNOLD!!!!," Stallone wrote. "As long as you live and beyond, you're always going to be the 'BIG MAN' who set the bar so high that it will never be surpassed, an action hero legend!"
That's high praise given Stallone's own action resume, but one fit for the Terminator star.
Stallone and Schwarzenegger have starred in several films together, including all three Expendables films and 2011's Escape Plan.
The two iconic actors are good friends with high opinions of each others' works, so much so that Schwarzenegger has said that he wouldn't return to the Expendables franchise without Stallone. Stallone is reportedly not planning to return to the series.
"There is no Expendables without Sly," he said earlier this year. "I would never do the movie without him, no... I think that Sly has good ideas, what he wants to do with it, and I think if they write a really good script [in] which my part is very well-developed, I would do it. If not, then I won't do it."