the prophet pbuh was well-known among his people for honesty before his prophethood, and used to be called "al-ameen" (the trustworthy). this is a name that would not be given except to one who was extremely truthful, trustworthy and upright.
even his enemies testified to this. abu jahl, in spite of his hatred for the prophet pbuh and denial of him, knew that he was truthful. a man asked him, "is muhammad truthful or is he a liar?" he replied, "woe to you! by allah, muhammad is truthful and he has never told a lie. but, if the sons of qusayy acquire the standard, the distribution of water for pilgrims, the custodianship of the ka`bah and prophethood, what will be left for the rest of the quraysh?"
and abu sufyan, who before accepting islam was the most adamant enemy of the prophet pbuh, was asked by heraclius, "did you ever accuse him of lying before he said what he said?" abu sufyan said, "no." so heraclius concluded, "i knew from what you told me that he would not have refrained from lying to people and then lie about allah the almighty."
and his wife, khadijah may allah be pleased with her, when the prophet pbuh came back to her, trembling and saying, "wrap me up," after revelation first descended upon him in the cave of hiraa', said to him, "good tidings! no, by allah, allah will never humiliate you. for you keep ties with relatives and you speak the truth..." (al-bukhari and muslim)
ibn `abbas may allah be pleased with him reported: "after the verse 'and warn your closest kinsmen' was revealed, the messenger of allah pbuh went out, climbed the hill of safa and shouted to get their attention. they said, "what's this?" and gathered before him. he addressed them, saying,
"if i was to inform you that horsemen were in the valley behind this hill ready to attack you, would you believe me?"
they replied, "yes, for we have never known you to lie". he pbuh said,
"indeed, i am a warner to you of severe punishment." (al-bukhari and muslim)
the honesty and integrity of the prophet caused the polytheists confusion over how to describe him. sometimes they would say he was a lying magician, sometimes a poet, sometimes a soothsayer and sometimes that he was mad; and they would express disapproval of one another over this because they all knew that these were not attributes of the prophet pbuh.
an-nadr ibn al-harith, who severely abused the prophet, said to the quraysh, "you are being afflicted with something the likes of which you have never experienced. as a young man, muhammad was the most intelligent, truthful and trustworthy among you. but when you noticed grey hairs at his temples and he conveyed to you what he conveyed, you called him a magician. no, by allah, he is not a magician. and you called him a soothsayer. no, by allah, he is not a soothsayer. and you called him a poet. no, by allah, he is not a poet. and you said he was mad... o company of quraysh! look into this matter of yours, for by allah, you have been afflicted with something tremendous."
the prophet's integrity was the actual cause of khadijah's wish to be his wife. he was in charge of her business transactions in syria and she learned from her servant, maysarah, of his honesty and noble manners.
the greatest trust carried by the prophet pbuh and fulfilled by him to the utmost degree was that concerning the revelation and the message that allah the almighty had charged him with conveying to the people. the prophet conveyed it and fulfilled his trust in the most complete and perfect way. he pbuh opposed the enemies of allah the almighty with irrefutable evidence and then by the sword, so allah granted him conquests and opened hearts to his invitation. people believed in him, aided and supported him until the word of tawheed (la ilaha ill-allah) became dominant and islam spread eastward and westward. there remained not a single house but that allah the almighty had caused his religion to enter it.
may the blessings and peace of allah be upon the truthful and trustworthy prophet, who strove for the cause of allah with the striving it deserved until he was finally taken by death.