Sahih al-Bukhari 2139
Sahih al-Bukhari · Book 34: Sales and Trade · Not to cancel a bargain already agreed upon
Narrated `Abdullah bin `Umar:
حَدَّثَنَا إِسْمَاعِيلُ ، قَالَ حَدَّثَنِي مَالِكٌ ، عَنْ نَافِعٍ ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لاَ يَبِيعُ بَعْضُكُمْ عَلَى بَيْعِ أَخِيهِ " .
Allah's Messenger (ﷺ) said, "Do not urge somebody to return what he has already bought (i.e. in optional sale) from another seller so as to sell him your own goods."