The true investor scarcely ever has to sell his shares, and at all other times he is free di disregard the current price quotation. He need pay attention to it and act upon it only to the extent that it suits his book, and no more. Thus the investor who permits himself to be stampeded or unduly worried by unjustified market declines in his holdings is perversely transforming his basic advantage into a basic disadvantage. That man would be better off if his stocks had no market quotation at all, for he would then be spared the mental anguish caused him by other persons' mistakes of judgement.

Chapter II, The Investor and Stock-Market Fluctuations, p. 40 - The Intelligent Investor: The Classic Text on Value Investing (1949)