From Ibn 'Umar, that the Messenger of Allah ﷺ said: "Both the buyer and the seller retain the option as long as they have not separated or they give each other than option." He (Nafi') said: "So when Ibn 'Umar purchased something while he was sitting, he would stand to complete the sale."
سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ الْبَيِّعَانِ بِالْخِيَارِ مَا لَمْ يَتَفَرَّقَا أَوْ يَخْتَارَا قَالَ فَكَانَ ابْنُ عُمَرَ إِذَا ابْتَاعَ بَيْعًا وَهُوَ قَاعِدٌ قَامَ لِيَجِبَ لَهُ الْبَيْعُ