Sahih Muslim 1515 c
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.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏
"‏ لاَ يُتَلَقَّى الرُّكْبَانُ لِبَيْعٍ وَلاَ يَبِعْ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ وَلاَ تَنَاجَشُوا وَلاَ يَبِعْ حَاضِرٌ لِبَادٍ وَلاَ تُصَرُّوا الإِبِلَ وَالْغَنَمَ فَمَنِ ابْتَاعَهَا بَعْدَ ذَلِكَ فَهُوَ بِخَيْرِ النَّظَرَيْنِ بَعْدَ أَنْ يَحْلُبَهَا فَإِنْ رَضِيَهَا أَمْسَكَهَا وَإِنْ سَخِطَهَا رَدَّهَا وَصَاعًا مِنْ تَمْرٍ ‏"‏ ‏.‏