Namely, the following one

"All problems appeared once in the [American Mathematical]

Monthly."

I remember reading it several years ago... When I first posed the question, I believed that I had read it somewhere in Krantz' *Mathematical Apocrypha* but according to Carlo Beenakker the quote is nowhere to be found in the said recollection of stories and anecdotes of Krantz. I gather that Erdős might have expressed it in relation with the history of what is nowadays known as the Sylvester-Gallai theorem but it'd be perfectly possible for me to be wrong in this respect too.

I thank you in advance for your replies.