It was narrated from Ibn Juraij: "Nafi dictated to me, from Ibn 'Umar who said: The Messenger of Allah (ﷺ) 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 the transaction. If they have both chosen to conclude the transaction, then the transactions binding." (Sahih )
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عَلِيُّ بْنُ مَيْمُونٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ ابْنِ جُرَيْجٍ، قَالَ أَمْلَى عَلَىَّ نَافِعٌ عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا تَبَايَعَ الْبَيِّعَانِ فَكُلُّ وَاحِدٍ مِنْهُمَا بِالْخِيَارِ مِنْ بَيْعِهِ مَا لَمْ يَفْتَرِقَا أَوْ يَكُونَ بَيْعُهُمَا عَنْ خِيَارٍ فَإِنْ كَانَ عَنْ خِيَارٍ فَقَدْ وَجَبَ الْبَيْعُ " .
sunnah.com