Sahih Bukhari Volume 3, Book 34, Hadith 2151

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكٌ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ لاَ تَلَقَّوُا الرُّكْبَانَ، وَلاَ يَبِيعُ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ وَلاَ تَنَاجَشُوا وَلاَ يَبِيعُ حَاضِرٌ لِبَادٍ، وَلاَ تُصَرُّوا الْغَنَمَ، وَمَنِ ابْتَاعَهَا فَهْوَ بِخَيْرِ النَّظَرَيْنِ بَعْدَ أَنْ يَحْتَلِبَهَا إِنْ رَضِيَهَا أَمْسَكَهَا، وَإِنْ سَخِطَهَا رَدَّهَا وَصَاعًا مِنْ تَمْرٍ ‏"‏‏.‏

Narrated By Abu Huraira : Allah's Apostle said, "Do not go forward to meet the caravan (to buy from it on the way before it reaches the town). And do not urge buyers to cancel their purchases to sell them (your own goods) yourselves, and do not practice Najsh. A town dweller should not sell the goods for the desert dweller. Do not leave sheep un-milked for a long time, when they are on sale, and whoever buys such an animal has the option of returning it, after milking it, along with a Sa of dates or keeping it. it has been kept un-milked for a long period by the seller (to deceive others).