Sahih Muslim 10960
In-book no 3683حَدَّثَنَا 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 " .
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.