Sincerity is the way of heaven. Being honest to others is a principle for conducting oneself.
诚者，天之道也;诚之者,人之道也.

If you are not sincere you cannot touch others.
不精不诚，不能动人

In contact with friends one should keep one's word.
与朋友交，言而有信。

How can one conduct himself in society if he does not keep his word?
人而无信不知其可也。

A word once spoken cannot be taken back even by a team of four horses.
一言既出，驰马难追。