The religion of Islam is a religion based on justice, and doing justice is the basis of faith, because it is what Allah (swt) has commanded:

 إِنَّ اللَّهَ يَأْمُرُكُمْ أَن تُؤَدُّوا الْأَمَانَاتِ إِلَىٰ أَهْلِهَا وَإِذَا حَكَمْتُم بَيْنَ النَّاسِ أَن تَحْكُمُوا بِالْعَدْلِ ۚ إِنَّ اللَّهَ نِعِمَّا يَعِظُكُم بِهِ ۗ إِنَّ اللَّهَ كَانَ سَمِيعًا بَصِيرًا

(Indeed, Allah commands you to render trusts to whom they are due and when you judge between people to judge with justice. Excellent is that which Allah instructs you. Indeed, Allah is ever Hearing and Seeing.) (An-Nisaa: 58)

Justice must prevail in every matter so that rights are not lost, and so that every person obtains his right and performs his duty in the best way.

(… يَا أَيُّهَا الَّذِينَ آمَنُوا كُونُوا قَوَّامِينَ بِالْقِسْطِ ) (O you who have believed, be persistently standing firm in justice …): Meaning, you who have believed in Allah (swt) be standing firm in justice not only one time but always in every affair in your life and in every matter.

(… شُهَدَاءَ لِلَّهِ وَلَوْ عَلَىٰ أَنفُسِكُمْ أَوِ الْوَالِدَيْنِ وَالْأَقْرَبِينَ …) (…witnesses for Allah, even if it be against yourselves or parents and relatives.…): That is, when a person witnesses for Allah (swt) against himself that he is the one who committed something or against the closest people to him, the testimony must certainly be with justice, and our Prophet Muhammad (saws) taught us that through the story of the Makhzumi woman when he said:

“By the One in whose hand is the soul of Muhammad, if Fatimah bint Muhammad were to steal, I would cut off her hand.”[1]

So the Muslim has to be persistently standing firm in justice even against himself because this is an admission of guilt and is the first way to repent to Allah (swt).

(… إِن يَكُنْ غَنِيًّا أَوْ فَقِيرًا فَاللَّهُ أَوْلَىٰ بِهِمَا …) (…Whether one is rich or poor, Allah is more worthy of both.…): Someone may deviate from justice out of greed for money, out of fear of the authority of the rich, or as mercy and compassion for the poor. Allah (swt) commands you to be just in all cases. Wealth and poverty have nothing to do with the issue of justice. Justice is an issue of rights, and the lives of societies are not upright except when rights are fulfilled and justice is the basis.

(… فَلَا تَتَّبِعُوا الْهَوَىٰ أَن تَعْدِلُوا …) (…So follow not [personal] inclination, lest you not be just. …): Because unfairness means following a whim, either this whim is misguidance, or it is out of greed for money, bribery, or fear. Whatever it is, do not follow whims to avoid being unjust.

( وَإِن تَلْوُوا أَوْ تُعْرِضُوا فَإِنَّ اللَّهَ كَانَ بِمَا تَعْمَلُونَ خَبِيرًا … ) (…And if you distort [your testimony] or refuse [to give it], then indeed Allah is ever, with what you do, Acquainted): It suffices for you that Allah (swt) is All-Knowing of what  you did in the lack of justice and the adoption of perjury, lying, or other things.

It was narrated that one of the judges had a case of contention between two opponents and he loved dates very much, especially in his first days, so one of them came and knocked on the door of his house a day before the date of the court session and he had a basket of dates. The judge refused to take it and closed the door in his face even though he loves them very much. Next day was the session in the judiciary, so that man came with his opponent to the judge and stood in front of the judge, so he ruled with the truth, but when he came out of the courthouse he went to Amir al-Muminin and said: O Chief of Believers, by Allah, they are not equal in my view, so I submit my resignation from the judiciary, so the Caliph was surprised and he asked him: What are you talking about, our judge? He replied: “O Chief of Believers, there is a case of disagreement between two people, so one of them came to my house and offered me dates, although I refused them, but when they stood before me in the courthouse, by Allah, they were not equal for my heart inclined to one of them, although I judged justly for the one who brought me the dates, so I submit my resignation from the judiciary.” This is the justice that the verse talks about, and which the Prophet (saws) established, and the honorable companions were brought up on it, so they established justice in ruling between the disputants and spread it

[1] Sahih al-Bukhari: Book of: Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi), Chapter of: Who witnessed Makkah conquest, Hadith No. 4053.