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Surah 11. Hud
Ayah 85 - 86
85وَيَـٰقَوْمِ أَوْفُوا۟ ٱلْمِكْيَالَ وَٱلْمِيزَانَ بِٱلْقِسْطِ ۖ وَلَا تَبْخَسُوا۟ ٱلنَّاسَ أَشْيَآءَهُمْ وَلَا تَعْثَوْا۟ فِى ٱلْأَرْضِ مُفْسِدِينَ
86بَقِيَّتُ ٱللَّهِ خَيْرٌ لَّكُمْ إِن كُنتُم مُّؤْمِنِينَ ۚ وَمَآ أَنَا۠ عَلَيْكُم بِحَفِيظٍ
85. "And O my people! Give full measure and weight in justice and reduce not the things that are due to the people, and do not commit mischief in the land, causing corruption.'' 86. "That which is left by Allah is better for you, if you are believers. And I am not a guardian over you.''
First, he (Shu`ayb) prohibited them from cheating in business by decreasing the weights whenever they gave (products) to people.
He commanded them to give just measure and weight whether they were giving or receiving (in transactions). He also forbade them from causing mischief and corruption in the land. This was due to their practice of highway robbery along the roads. Abu Ja`far bin Jarir said,
(That which is left by Allah (after giving the rights of the people) is better for you,) "This means what you gain from your successful business dealings in which you have given just measure, is better for you than wrongfully taking the wealth of the people.'' Ibn Jarir said that this statement has been reported from Ibn `Abbas and I say it is similar to Allah's statement,
(Say: "Not equal are Khabith (all that is evil) and Tayyib (all that is good), even though the abundance of Khabith may please you.'')﴿5:100﴾ Allah then says,
(And I am not a guardian over you.) This means a watcher over you people. In other words, "Do this for Allah and not to be seen by the people.''