Where in the Bible does it say to thine own self be true?

Where in the Bible does it say to thine own self be true?

It occurred to me that the Bible might have guidance on the subject, and I found these words of Christ Jesus: "I can of mine own self do nothing: as I hear, I judge: and my judgment is just; because I seek not mine own will, but the will of the Father which hath sent me" (John 5:30).May 25, 2007

Who said the quote to thine own self be true?

Polonius