It's dattebayo. Naruto ends sentences that way, because in Japan it seems that depending on who you are and other things you end the sentences with different words. For example, girls often end sentences with 'yo', like Sakura does. Cool guys end it with 'da', like Sasuke. Naruto does 'dattebayo' because it's a word that shows he's a little trickster or whatnot. I hope this answers your question.

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