I think that I once read something attributed to Rashi that was along the lines of "Any person can say that something is forbidden, but it takes a great person to say that it is permitted". Does anyone know where this is from?
Maybe you're referring to one of the many times Chazal employ the concept of כח דהתירא עדיף, it's more substantial to permit something (than to forbid something). Rashi usually explains it in the manner you quote. For example, Beitzah 2b:
דהתירא עדיף ליה - טוב לו להשמיענו כח דברי המתיר שהוא סומך על שמועתו ואינו ירא להתיר אבל כח האוסרין אינה ראיה שהכל יכולין להחמיר ואפילו בדבר המותר:
He'd prefer to teach us the strength of the one who permits, who is relying on his tradition, and isn't afraid to permit. However, the strength of those who forbid isn't a proof of anything, since anyone can be stringent, even with something in fact permitted.