Sahih Muslim Book 10, Hadith Number 3627.

Chapter : It is forbidden to meet the traders in the way for getting undue advantage.

Abu Huraira (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as saying: Do not meet the merchant in the way and enter into business transaction with him, and whoever meets him and buys from him (and in case it is done, see) that when the owner of (merchandise) comes into the market (and finds that he has been paid less price) he has the option (to declare the transaction null and void).