Hadith.3982 - Imam Jafar ibn Muhammad Al-Sadiq (as) said: "Defrauding the one who entrusts you is unlawful gain, and defrauding a believer is forbidden."
Hadith.3984 - Imam (as) said: "If a man says to another man, 'Come, I will improve your sale,' then it is forbidden for him to make a profit."
[REFERENCES]
Al-Kafi, Vol.5 p.152 • Man La Yahduruhu Al-Faqih, Vol.3 p.272 • Tahdhib Al-Ahkam, Vol.7 p.7 • Al-Wafi, Vol.17 p.457 • Wasa'il Al-Shi'ah, Vol.17 p.395 • Bihar Al-Anwar, Vol.100 p.136