• Evidence from the Qur’an
Almighty Allah says,
يَا أَيُّهَا الَّذِينَ آمَنُوا لا تَأْكُلُوا أَمْوَالَكُمْ بَيْنَكُمْ بِالْبَاطِلِ إِلا أَنْ تَكُونَ تِجَارَةً عَنْ تَرَاضٍ مِنْكُمْ وَلا تَقْتُلُوا أَنْفُسَكُمْ إِنَّ اللَّهَ كَانَ بِكُمْ رَحِيمًا (٢٩)
“O you who believe! Do not devour your property among yourselves falsely, except that it be trading by your mutual consent; and do not kill your people; surely Allah is Merciful to you.” Surat An-Nisaa’, 4:29
In this verse, Almighty Allah forbids devouring people’s property falsely unless they are aware of all trading activities and mutually consent to them. There is no doubt that engaging in deceptive practices such as manipulating prices and tricking fellow traders in the stock market is the worst form of deceiving them and devouring their property falsely without their consent. See Badaa’i’ As-Sanaa’i’, 5/274; Al-Qurtubee, Al-Jaami’ Li Ahkaam Al-Qur’aan, 2/225; and Ibn Taymiyyah, Majmoo’ Al-Fataawaa, 15/127 and 28/104.
The Qur’an also says,
وَإِلَى مَدْيَنَ أَخَاهُمْ شُعَيْبًا قَالَ يَا قَوْمِ اعْبُدُوا اللَّهَ مَا لَكُمْ مِنْ إِلَهٍ غَيْرُهُ قَدْ جَاءَتْكُمْ بَيِّنَةٌ مِنْ رَبِّكُمْ فَأَوْفُوا الْكَيْلَ وَالْمِيزَانَ وَلا تَبْخَسُوا النَّاسَ أَشْيَاءَهُمْ وَلا تُفْسِدُوا فِي الأرْضِ بَعْدَ إِصْلاحِهَا ذَلِكُمْ خَيْرٌ لَكُمْ إِنْ كُنْتُمْ مُؤْمِنِينَ (٨٥)
“And to Madyan [We sent] their brother Shu’ayb. He said: O my people! worship Allah, you have no god other than Him; clear proof indeed has come to you from your Lord; therefore, give full measure and weight and do not diminish to men their things, and do not make mischief in the land after its reform; this is better for you if you are believers.” Surat Al-A’raaf, 7.85.
The Arabic word bakhs (translated here as ‘diminish’) signifies ‘decrease, deception, tricking others as to the real value of the commodity or using tricks to receive a full measure or giving others less…” Ibn Al-‘Arabee, Ahkaam Al-Qur’aan, 2/788
Taghreer includes all these deceptive practices, hence the prohibition.