I was looking for the exact quote "Know thyself..." while researching for by blog about the sleep talking man.

I found several interesting pages that need to be sorted out and integrated in the wiki:

Proverbs From the Ancient Egyptian Temples
http://www.duboislc.org/html/Proverbs.html

Someone once said "Know Thyself". Is This Important: What Does it Mean
http://hubpages.com/forum/topic/32889
which includes this fascinating reply:

"Gnothi seauton"

It was inscribed in gold at the entrance to the temple of Apollo at Delphi.

In 512 BC, Pythagoras wrote: "Man Know thyself and thou shalt know the universe and god."

Thales of Miletus (approx. 625-545 BC)when asked what was most difficult, answered,"to know one's self ",

Clement of Alexandria (150-215) said, "The greatest of all lessons is to know oneself; for if someone knows himself, he will know God; and if he knows God, he will become like God."

The Gospel of Thomas from Nag Hammadi has Jesus saying "He who has not known himself has known nothing, but he who has known himself has at the same time already achieved knowledge about the depth of all."

And Buddha (568-488 BC) said "Look within Thou art the Buddha"

wikipedia:
http://en.wikipedia.org/wiki/Know_thyself