Sunan an-Nasa'i 4472
It was narrated from Al-Laith, from Nafi, from Ibn 'Umar that the Messenger of Allah said:
"When two men enter into a transaction, each of them has the choice until they separate."  On one occasion he said: "So long as they have not separated and one has not told the other to decide. If one tells the other to decide and they agree upon something, then the transaction is binding. If they separate after entering into a transaction and neither of them has canceled the transaction, then the transaction is binding." (Sahih )
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ إِذَا تَبَايَعَ الرَّجُلاَنِ فَكُلُّ وَاحِدٍ مِنْهُمَا بِالْخِيَارِ حَتَّى يَفْتَرِقَا ‏"‏ ‏.‏ وَقَالَ مَرَّةً أُخْرَى ‏"‏ مَا لَمْ يَتَفَرَّقَا وَكَانَا جَمِيعًا أَوْ يُخَيِّرَ أَحَدُهُمَا الآخَرَ فَإِنْ خَيَّرَ أَحَدُهُمَا الآخَرَ فَتَبَايَعَا عَلَى ذَلِكَ فَقَدْ وَجَبَ الْبَيْعُ فَإِنْ تَفَرَّقَا بَعْدَ أَنْ تَبَايَعَا وَلَمْ يَتْرُكْ وَاحِدٌ مِنْهُمَا الْبَيْعَ فَقَدْ وَجَبَ الْبَيْعُ ‏"‏ ‏.‏