Leopold von Ranke Quote

[Jona Lendering]

In another thread, I just referred to that famous remark by Leopold von Ranke that as a historian, he only knew the upper part of people; what happened below the waste, was not his business ("Als Historiker kenne ich die Menschen nur oberhalb des Nabels; was unterhalb sich befindet, geht mich nichts an.").

Actually, I can not back up my quote. Does anyone know the source?