Richest - but since when?

[Olaf Solstrand]

I've started pondering over a question...

What was the FIRST story where it was stated that Scrooge is the richest 
man/duck in the world?

I know the fact that he is very, very rich was established pretty early, but 
what I ponder about is when it was stated that he's _the richest_.

Can anyone help me on that one? I _assume_ it's a Barks story (seems very 
logical to me), and my guess is that it's US 15 "The Second-richest Duck" - but 
I'm not very sure about that. In fact, I'm not sure at all.