Sahih (Sahih Muslim)
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 " .
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