Before playing the "show me a link" game, you should probably do some basic research. He doesn't need a "source" since it is apparent to anyone who knows anything about the process.
Impeachment is a purely political process. There is no judicial review of impeachment on articles of conviction, and indeed, the constitution doesn't even bother to spell out "high crimes and misdemeanors". It's a catch-all.
But really, all you need to know is that there is no judicial review. Because there is no judicial review, a president can be impeached for having an ugly face, if congress really was willing to vote to convict on ugly face impeachment articles. You could jump up and down screaming about it, but it wouldn't make a difference.
:shrug:
Now if you really do want to dig further and if you look in the right places, you will find a number of pretty strong arguments about how the founders appeared to intend it to relate to crimes of corruption related to the office. ie, in obtaining the office or committed via use of office. But again, they didn't bother to define it in the constitution and they didn't grant judicial review, so it can be anything congress wants it to be.
And don't bother demanding a link from me. I don't play that game. People pretty much never actually read then honestly discuss a demanded link. It's just another boring debate board move.