All praise be to Allah, and peace and blessings be upon the Messenger of Allah, and his family, Companions, and followers.
Allah (Glorified and Exalted be He) makes it obligatory on Muslims to be truthful and sincere in all their dealings and prohibits them from lying, cheating, and deceiving others. Honesty, truthfulness, and keeping trusts serve the interests of society, promote proper cooperation between Muslims, and protect them from committing injustices or wronging one another. On the other hand cheating, deceit, and lying lead to the spread of corruption, injustice, usurpation of property in society and to feelings of enmity and hatred.
It is authentically reported that the Messenger of Allah (peace be upon him) said, "The Din (religion) is sincerity." They said, "To whom, O Messenger of Allah?" He (peace be upon him) answered, "To Allah, His Book, His Messenger, and the Imams (leaders) of Muslims and their common people." (Related by Muslim in his "Sahih [Book of Authentic Hadith]")
It is also related in the Two Sahih (authentic) Books of Hadith (i.e. AlBukhari and Muslim)on the authority of Jarir ibn 'Abdullah AlBajaly (may Allah be pleased with him) who narrated, "I pledged allegiance to the Prophet (peace be upon him) based on the establishment of the Salah (Prayer), the payment of Zakah (obligatory charity), and sincerity to every Muslim."
It is also related in the Two Sahih (authentic) Books of Hadithon the authority of Hakim ibn Hizam (may Allah be pleased with him) who narrated that the Messenger of Allah (peace be upon him) said, "Both parties in a business transaction have the choice (either to annul or conclude the transaction), as long as they have not separated." According to another narration, he (peace be upon him) said, "...until they separate. If they are honest and disclose any defects, they will be blessed in their transaction; but if they conceal defects and lie, the blessing of their transaction will be eradicated."
It is authentically reported that the Prophet (peace be upon him) said, "Anyone who cheats us is not one of us."
It is related in: "Sahih Muslim" on the authority of Abu Hurayrah (may Allah be pleased with him) who narrated, "The Prophet (peace be upon him) passed by a pile of foodstuff. He put his hand into it and his fingers got moistened. He (peace be upon him) said, ' What is this, O owner of the foodstuff?' He replied, 'It got rained on, O Messenger of Allah.' He (peace be upon him) said, 'Why do you not put it on the top of food so that people can see it? Anyone who cheats is not of me (my followers).'"
All of these Sahih (authentic) Hadith and those that hold their meanings are evidence of the obligation of sincerity, transparency, and honesty in transactions and stress the prohibition of lying, cheating and deceit.