出典を教えてください

I've tried to find this sentence in the 'question box of English', but
it's too many to go on searching it. If there's any function to find it. please teach me.

This is not a piece of advice to you I'm talking to myself for my practice of writing skills.
I found this sentence in the wblio dictionary and on the web, but I can't tell the source of it.
https://ejje.weblio.jp/sentence/content/discoverers
https://dict.longdo.com/?page=popthai&search=As%20a%20matter%20of%20fact,%20all%20great%
20discoverers%20have%20been%20regarded%20as%20dreamers.