He also said:

Section

Truthfulness is the foundation of good deeds and what unites them; lying is the foundation of evil deeds and what binds them together. This becomes clear in several ways:

First: The human being is a living being endowed with speech. Speech is the defining characteristic that distinguishes humans from other animals. Speech is of two kinds: declarative and non-declarative. A statement is sound when truthful and corrupt when false. Thus, a liar is in a worse condition than a speechless beast. Declarative speech distinguishes the human being and is the basis of non-declarative speech: the former expresses knowledge, while the latter expresses action. Knowledge precedes action and gives rise to it. The liar has not merely lost what makes him human; he has turned it into its opposite. Hence the saying: “A habitual liar has no nobility of character; an envious person has no peace; there is no brotherhood with kings; and a miser has no eminence.” For murū'a (nobility of character) is the abstract noun corresponding to mar' (a person), just as “humanity” corresponds to “human.”

Second: What distinguishes a prophet from a false claimant to prophethood is truthfulness as opposed to lying.