That's Cervantes? I thought it was by Leigh and Darion. It's possible that they adapted a real (translated) Cervantes quote for a song in Man of La Mancha, but a few Googles don't yield any trustworthy sources that attribute the quote to Cervantes himself.

In any event, Brian Stokes Mitchell did a phenomenal rendition: https://www.youtube.com/watch?v=zbgTUwUP-ew

Wait, this is the Broadway World forum, right?

Edit: Yup. Confirmed with someone with a doctorate in Spanish Lit and a deep love of Cervantes. That quote was invented for the musical. Oh well.