Al-Muwatta Hadith 31.97

Malik related to me from Abu'z-Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "Do not go out to meet the caravans for trade, do not bid against each other, outbidding in order to raise the price, and a townsman must not buy on behalf of a man of the desert, and do not tie up the udders of camels and sheep so that they appear to have a lot of milk, for a person who buys them after that has two recourses open to him after he milks them. If he is pleased with them, he keeps them and if he is displeased with them, he can return them along with a sa of dates." Malik said, "The explanation of the words of the Messenger of Allah, may Allah bless him and grant him peace, according to what we think - and Allah knows best - 'do not bid against each other,' is that it is forbidden for a man to offer a price over the price of his brother when the seller has inclined to the bargainer and made conditions about the weight of the gold and he has declared himself not liable for faults and such things by which it is recognised that the seller wants to make a transaction with the bargainer. This is what he forbade, and Allah knows best." Malik said, "There is no harm, however, in more than one person bidding against each other over goods put up for sale." He said, "Were people to leave off haggling when the first person started haggling, an unreal price might be taken and the disapproved would enter into the sale of the goods. This is still the way of doing things among us."