From Xunzi's (
荀子, Warring States)
《荀子·劝学》 ('Encouraging Learning'):
木受绳则直,金就砺则利,君子博学而日参省乎己,则知明而行无过矣 - 'wood is straightened by the line, metal is sharpened by the whetstone; the noble one studies widely and examines himself daily, so that his knowledge is clear and his conduct without fault.' The founding text of the Confucian case for relentless self-cultivation.