The Holy Prophet (S) said: "He who purchases the food stuff which is a public necessity and stores it for forty days with the hope that its price will go up in the Muslim market, then, sells it and gives the entire sum as charity to the poor, it can not be the atonement for what he has done."
Bihar-ul-Anwar, vol. 103, p. 89
Imam Amir ul Mu'mineen ‘Ali (‘a) said: "He who ventures transactions without observing the concerning Islamic rules, has surely run into usury," (while he is not aware of it).
Bihar-ul-Anwar, vol. 103, p. 93
Imam Amir ul Mu'mineen ‘Ali (‘a) narrates from the Messenger of Allah who said: "He who is in business and buys and sells things must avoid five traits, otherwise he should not buy or sell anything: usury, taking oath, concealing the faults or defects of the goods, praising it wrongly when selling it, and finding faults in it when buying it."
Al-Khisal by Saduq, vol. 1, p. 286
Imam al-Baqir, the fifth Imam (‘a) said: "He who seeks for sustenance in this world in order to be independent of people for his needs, to provide for his famlly members, and to stretch affection unto his neighbours, will meet Allah, Almighty and Glorious, on the Day of Judgement while his face will be as bright as the full moon."
Al-Kafi, vol. 5, p. 78