a'ishah, with whom allah is pleased, said:

'the trait and characteristic which the prophet, may allah exalt his mention, hated most was lying. a man would tell a lie in the presence of the prophet, may allah exalt his mention, and he would hold it against him, until he knew that he repented.' (tirmidthi 1973)

even his enemies attested to his truthfulness. abu jahl, who was one of the harshest enemies, said: ‘o muhammad! i do not say that you are a liar! i only deny what you brought and what you call people to.’ allah, the exalted, says:

(we know indeed that what they say certainly grieves you, but surely they do not call you a liar; but the unjust deny the verses of allah.)  [6:33]