Sahih Muslim Book 010, Hadith Number 3621.

Chapter : It is forbidden that one should enter into a transaction on which one's brother has already been negotiating, or one should purchase (in opposition) to one's brother, or one should cheat and retain milk in the under (in order to deceive the purchaser).

Abu Huraira (Allah be pleased with him) reported that Allah's Messenger (may peace be upon him) forbade the (people) meeting the caravan (for entering into business transaction with them), and the selling of goods by a townsman on behalf of a man of the desert, and seeking by a woman the divorce of her sister (from her husband), and outbidding (against one another), and tying up the udders (of animals), and buying of (things) in opposition to one's brother. This hadith has been narrated through another chain of transmitters.

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