Long before the Prophet Muhammad was appointed a prophet, he was given the epithet" The Honest" by his enemies

Muhammad: The Honest

An epithet, linguistically speaking, is "A term used to characterize a person or thing, such as the Great in Catherine the Great"; or

"A term used as a descriptive substitute for the name of a person, such as The Great Emancipator for Abraham Lincoln (The American Heritage Dictionary).

"The Honest" has likewise been the most outstanding epithet given to the Prophet Muhammad (pbuh) by his early-life people; it is an indubitable proof of honesty and ethicality which has even made biased orientalists like Montgomery Watt say:

"By his contemporaries, however, Muhammad was admired for his courage, resoluteness, and impartiality, and for a firmness that was tempered by generosity. He won men's hearts… an exemplar of virtuous character."

Once the masters of his tribesmen fought for who should set the honorable black stone of theKaaba in its former place, after it had been about to collapse and agreed to be rebuilt. They came to terms that the first passerby should arbitrate the dispute between them. It was Muhammad (pbuh) that passed by then when all disputants cried: "This is the Honest, This is Muhammad, we agree to him" ("The Sealed Nectar").

That is why they always kept their valuables with him when traveling. He was famous for it even after being revealed to from God and antagonized by them.

His honesty has not only been manifest with people but with God in the first place too. There are Quranic verses wherein God blames the Prophet for the purpose of instruction; if wasn't sincere, argumentatively speaking, he wouldn't have said them (consider Quranic Interpretations like At-Tabary's: chapters 33 (verse 37) and 80 (verse 1).

In significant Quranic verses, I notice, God combines between Gabriel's epithet of honesty and the Prophet's heart:

"And verily it is a revelation… (192) Which the Honest Spirit has brought down (193) Upon your heart…" ('The Poets').