Sunan an-Nasa'i 4470
It was narrated from Ayyub, 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 chosen to conclude the transaction." Or perhaps Nafi said: "Or one of them has said to the other: 'Decide! "(Sahih )
أَخْبَرَنَا زِيَادُ بْنُ أَيُّوبَ، قَالَ حَدَّثَنَا ابْنُ عُلَيَّةَ، قَالَ أَنْبَأَنَا أَيُّوبُ، 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 ‏"‏ ‏.‏