Is this a proverb?

We were walking along, when Mrs YesBut turned to me and said "If wishes were horses, man could fly".

I asked, "where did that come from?".

She said she didn't know.

Googled the phrase, while there are similar phrases, nothing exactly the same.

Can you help by providing an explanation?

I'm baffled - though that's nothing new, Mrs YesBut frequently confounds me.