The 'Terminator' star reached the milestone birthday on Sunday, and in a snippet of a speech given at the star's celebratory bash, fellow Hollywood veteran Sylvester, 71, took a moment to praise his "boundless energy".
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In the video clip posted on Instagram, Sylvester said: "You're so great, because you don't quit. You don't quit, and you still don't quit. And really it's boundless energy. But you've been a fantastic enemy, and a better friend, and that's all I can say."
And although the clip is only short, the 'Rocky' star assured fans it was only "a little bit" of his supposedly much lengthier speech to his pal.
He captioned the clip: "A little bit of my birthday speech to the Big Man ! It's actually a one of a kind competitive relationship which we are both grateful for! Keep punching , Arnold ! #arnoldschwarzenegger (sic)"
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Arnold hasn't yet posted anything on his own Instagram account to celebrate his special day, but earlier this month he did take to the photo sharing site to praise Sylvester when he turned 71 on July 6.
He wrote at the time: "Happy birthday to my dear friend, @officialslystallone. You deserve all of the celebration. Maybe we should take it easier next year? (sic)"
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Sylvester and Arnold have been close friends since starring together in the 'Expendables' movie franchise, and Arnold was recently seen sticking up for his pal when he said he wouldn't star in the fourth instalment of the series after Sylvester - who directed the first movie and starred in all three as Barney Ross - quit the franchise.
Arnold said at the time: "There is no 'Expendables' without Sly. I would never do the movie without him, no."