In the Holy Qur'an

O you who believe! be maintainers of justice, beareres of witness of Allah's sake, through it may be against your own selves or (your) parents or near relatives; if he be rich or poor, Allah is nearer to them both in compassion; therefore do not follow (your) low desires, lest you deviate, and if you swerve or turn aside, then surely Allah is aware of what you do. (4:135)

In the Traditions

The Prophet said, "Islami is Good Manners"

Imam Jafar said, "A Muslim is he whose hand and tongue people feel safe from, whilst a believer is he whom people trust with their belongings and their lives."