كتاب البيوع
21  
The Book of Transactions
(4)
Chapter: The prohibition of urging a buyer to cancel a purchase in order to sell him one's own goods; And urging a seller to cancel a sale already agreed upon so that one can buy the goods oneself; And the prohibition of artificially inflating prices; And the prohibition of letting milk accumulate in the udder in order to deceive the purchaser
(4)
باب تَحْرِيمِ بَيْعِ الرَّجُلِ عَلَى بَيْعِ أَخِيهِ وَسَوْمِهِ عَلَى سَوْمِهِ وَتَحْرِيمِ النَّجْشِ وَتَحْرِيمِ التَّصْرِيَةِ
Sahih Muslim 1412 e

Ibn 'Umar (Allah be pleased with them) reported Allah's Messenger (ﷺ) as having said this:

One amongst you should not enter into a transaction when another is bargaining.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ لاَ يَبِعْ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ ‏"‏ ‏.‏
Reference : Sahih Muslim 1412e
In-book reference : Book 21, Hadith 11
USC-MSA web (English) reference : Book 10, Hadith 3617
  (deprecated numbering scheme)