Injustice and Transgression
The Holy Prophet (S) said: "On the Day of the Resurrection a caller will announce: "Where are the oppressors and their assistants and those who prepared an inkwell for them or fastened a bag for them or supplied the ink of a pen (for them)? Then, gather these (people) with them!'"
Thawabab-ul-'A'mal, p. 309
Imam Amir ul Mu'mineen ‘Ali (‘a) said: "By Allah, even if I am given all the domains of the seven countries with all that exists under the skies in order that I may disobey Allah to the extent of snatching one grain of barley from an ant, I would not do it."
Nahjul-Balagha, p. 347
Imam al-Baqir, the fifth Imam, (‘a) said: "There are three types of transgressions: the one which Allah, the Exalted, forgives, the one He does not forgive, and that one which He does not ignore. Thus, the transgression that He does not forgive is infidelity unto Allah, the Almighty, and Glorious. And, the transgression which Allah forgives is the one that a person commits (against) himself between him and Allah, to Whom belong Might and Majesty. But the transgression which He does not ignore is the one committed against rights of men."
The third transgression is referring to when a person infringes upon the rights of others. The way to forgiveness is first by satisfying (contenting) the person whose rights were infringed upon. If that person forgives him, then the transgression becomes as one that the person has commited against himself. He may then seek forgiveness from Allah.
Al-Kafi, vol. 2, p. 330
Imam Amir ul Mu'mineen ‘Ali (‘a) said: "Transgression causes the foot to go astray, takes the blessings, and kills the nations."
Sharh Ghurar-ul-Hikam, vol. 2, p. 36
Imam Amir ul Mu'mineen ‘Ali (‘a) said: "... Nothing is more inducive of the reversal of Allah's bounty or for hastening of His retribution than continuance in oppression, because Allah hears the prayer of the oppressed and is on the look-out for the oppressors."
Nahjul-Balagha, Letter 53