Sunan an-Nasa'i 4479
It was narrated from Shu'ban who said:
"Abdullah bin Dinr narrated to us, from Ibn'Umar, who said; The Messenger of Allah said; "When two people meet to engage in trade, the transaction between them is not binding until they separate, unless    they have       chosen to conclude the transaction."
أَخْبَرَنَا عَمْرُو بْنُ يَزِيدَ، عَنْ بَهْزِ بْنِ أَسَدٍ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ دِينَارٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ كُلُّ بَيِّعَيْنِ فَلاَ بَيْعَ بَيْنَهُمَا حَتَّى يَتَفَرَّقَا إِلاَّ بَيْعَ الْخِيَارِ ‏"‏ ‏.‏