Almighty Allah says about dishonesty in the Holy Qur’an:1
And plead not on behalf of those who are dishonest to themselves. Surely, Allah loves not one who is perfidious and a great sinner.
According to a hadith:
Hazrat Abu Hurairah(ra) narrates that the Holy Prophet(sa) said, ‘If someone gives you something for safekeeping, return it to him. Do not be dishonest with anyone, even if he has been dishonest with you.’ (Sunano Abi Dawud, Kitab-ul-Buyu‘, Babu fir-Rajuli ya’khudhu Haqqahu…)
The Promised Messiah(as) says:
Of the various forms of discarding evil, the second is the virtue known as honesty and integrity, which is the indisposition to harm anyone by taking possession of his property mischievously and dishonestly. It should be clear that honesty and integrity constitute a natural human condition. That is why an infant, which is by nature simple and naïve, and, because of young age, has not yet acquired any bad habits, dislikes what belongs to others so much so that it is with great difficulty that [he] allows a strange woman to breastfeed [him]. (Islami Usul ki Philosophy, Ruhani Khaza’in, vol. 10, p. 344)
1 al-Nisa’, 4:108