Sunan an-Nasa'i 4471
It was narrated from Al-Laith from Nafi from Ibn 'Umar who said:
"The Messenger of Allah said: 'The two parties to a transaction both have the choice so long as they have not separated or they have chosen to conclude the transaction.' Or perhaps Nafi said: "Or one of them has said to the other: 'Decide! (Sahih )
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ الْبَيِّعَانِ بِالْخِيَارِ حَتَّى يَفْتَرِقَا أَوْ يَكُونَ بَيْعَ خِيَارٍ ‏"‏ ‏.‏ وَرُبَّمَا قَالَ نَافِعٌ ‏"‏ أَوْ يَقُولَ أَحَدُهُمَا لِلآخَرِ اخْتَرْ ‏"‏ ‏.‏