Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with the same chain of narration, it is narrated from (Sayyiduna Abu Hurairah, may Allah be pleased with him) that the Messenger of Allah, peace and blessings be upon him, said: "Do not go out to meet traders bringing goods from outside before they reach the market to buy their goods, and none of you should make a deal over the deal of his brother, and do not bid falsely (to deceive), and a townsman should not sell for a villager, and do not withhold milk in the udders of camels and goats (to deceive buyers). Then, if someone buys such an animal after that, he has two options after milking it: if he is pleased with it, he may keep the animal with him (and keep the deal), and if he dislikes it, he may return the animal along with a sa‘ (measure) of dates."