"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 ) (Using translation from Nasāʾī 4470)
قَالَ رَسُولُ اللهِ ﷺ الْبَيِّعَانِ بِالْخِيَارِ حَتَّى يَتَفَرَّقَا أَوْ يَكُونَ بَيْعَ خِيَارٍ قَالَ وَرُبَّمَا قَالَ نَافِعٌ أَوْ يَقُولَ أَحَدُهُمَا لِلْآخَرِ اخْتَرْ