Sahih Muslim | Book 10 | Hadith Number 3620

Narrated by Abu Huraira
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: Do not go out to meet riders to enter into transaction with them; none of you must buy in opposition to another, nor must you bid against one another; a townsman must not sell for a man from the desert, and do not tie up udders of camels and sheep, and he who buys them after that has been done has two courses open to him: after he has milked them he may keep them if he is pleased with them, or he may return them along with a sit of dates if he is displeased with them.