Actually, for Japanese, it has an error in grammar. In grammatical manner, it should be "だってよ(I heard...)" or "だってば(I swear that...) ". So, you might consider this expression as "I heard that .... , and I think it is definitely true!". Remember that no one except Naruto use this expression :p

= that's what he said = that's what I said (depending on context) だって = you know something from another source other than yourself