Sahih al-Bukhari 2112
Narrated Ibn `Umar:
Allah's Messenger (ﷺ) said, "Both the buyer and the seller have the option of canceling or confirming the 
bargain, as long as they are still together; and unless they separate or one of them gives the other the 
option of keeping or returning the things and a decision is concluded then, in which case the bargain is considered final. If they separate after the bargain and none of them has rejected it, then the bargain is rendered final."
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ قَالَ ‏
"‏ إِذَا تَبَايَعَ الرَّجُلاَنِ فَكُلُّ وَاحِدٍ مِنْهُمَا بِالْخِيَارِ، مَا لَمْ يَتَفَرَّقَا، وَكَانَا جَمِيعًا، أَوْ يُخَيِّرُ أَحَدُهُمَا الآخَرَ فَتَبَايَعَا عَلَى ذَلِكَ، فَقَدْ وَجَبَ الْبَيْعُ، وَإِنْ تَفَرَّقَا بَعْدَ أَنْ يَتَبَايَعَا، وَلَمْ يَتْرُكْ وَاحِدٌ مِنْهُمَا الْبَيْعَ، فَقَدْ وَجَبَ الْبَيْعُ ‏"‏‏.‏