Tao Teh Ching Chapter 27

善行无辙迹，善言无瑕谪；善数不用筹策；善闭无关楗而不可开，善结无绳约而不可解。
是以圣人常善救人，故无弃人；常善救物，故无弃物。是谓袭明。
故善人者，不善人之师；不善人者，善人之资。不贵其师，不爱其资，虽智大迷，是谓要妙。

Translation
The skilful traveller leaves no trials, the master speaker leaves no faults; He who knows how to count make no use of the abacus, he who knows how to lock make no use of bolts and yet it cannot be opened; He who knows how to bind make no use of strings or knot and yet it is impossible to unbind.
Thus the sage is skilful to bring salvation to people, and never abandons anyone; He is skilful to salvage even the lifeless objects, and never abandons anything. This is called inherited understanding.
Therefore the sage is the teacher of ordinary people, and the mundane are the subjects of the sage. If the ordinary one does not honor his guru, or the guru does not love his subjects, then even if both of them are intelligent, they are really ignorant. Remember, this is a very important principle.

Comments
The appearance never reveals what is really behind. Therefore to the normal person, a real master is hard to recognize. The skilful traveller and etc conceals their ability. The sage is responsible to the salvation of the world; however, they don’t appear to be doing it.
In the third paragraph, for the first time Lao Tzu mentions the relationship between a student and a guru. A student is 不善, or not kind, or not great. One cannot be a sage without realizing the truth, therefore all students are 不善. The dedication from a student to a guru and the love from a guru to a student is the most important thing in the student’s journey to enlightenment.