Same Authority that requires a President to be 35 years of age. The enforcement provisions of the Constitution are implicit, and need no enumeration in the text of the document.
You didn’t answer my question. I know what the constitution says. He was eligible. You haven’t shown any actual authority for your assertion. There’s no requirement that one affirmatively prove one’s citizenship. Trump didn’t do it. Neither did either Bush. I will grant you that the media couldn’t have cared less whether he was eligible, but there was never a sufficient challenge to any eligibility question.
He wasn’t qualified; he was beyond dangerous; everything else I could possibly say about him, well, isn’t very nice, to say the least. However, he WAS eligible as he was a US citizen.
You need to get over this since, even if you were correct, it doesn’t make a damn bit of difference now. Arguing about it is silly.