(4) باب تَحْرِيمِ بَيْعِ الرَّجُلِ عَلَى بَيْعِ أَخِيهِ وَسَوْمِهِ عَلَى سَوْمِهِ وَتَحْرِيمِ النَّجْشِ وَتَحْرِيمِ التَّصْرِيَةِ
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 carnels 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.
Reference: Sahih Muslim 1515 c
In-book reference: Book 21, Hadith 15
USC-MSA web (English) reference(deprecated numbering scheme): Book 10, Hadith 3620