it was narrated from Ismail, from Nafi, that Ibn 'Umar said:"The Messenger of Allah said: 'The two parties to a transaction both have the choice so long as they have not separated, unless they have both chosen to conclude they transaction. If they have both chosen to conclude the transaction, then the transaction is binding." (Sahih )
أَخْبَرَنَا مُحَمَّدُ بْنُ عَلِيٍّ الْمَرْوَزِيُّ، قَالَ حَدَّثَنَا مُحْرِزُ بْنُ الْوَضَّاحِ، عَنْ إِسْمَاعِيلَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الْمُتَبَايِعَانِ بِالْخِيَارِ مَا لَمْ يَفْتَرِقَا إِلاَّ أَنْ يَكُونَ الْبَيْعُ كَانَ عَنْ خِيَارٍ فَإِنْ كَانَ الْبَيْعُ عَنْ خِيَارٍ فَقَدْ وَجَبَ الْبَيْعُ " .
Nesai
The Book of Financial Transactions • Hadith 4467