definite article with the superlative degree of an adverb

[Natadtw1]

Hallo to everyone!

I would like to clarify the situation with the combination "the definite article + the superlative degree of an adverb".
E.g. He works hardest of all.
He works the hardest of all.

Which is correct?? I have seen both uses, Is there a rule regulating it?

Thank you.

[Beeesneees]

Both are acceptable but in most cases I would prefer use of the article.

'He works the hardest of all' - that is
He works the hardest out of all of them.
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Cheers m' dears!