I heard the Messenger of Allah ﷺ say, "The two parties to a sale each have the right to cancel as long as they have not parted or made their choice."So whenever Ibn Umar concluded a purchase while seated, he would stand up to make the sale binding.
حدثنا واصل بن عبد الأعلى حدثنا محمد بن فضيل عن يحيى بن سعيد عن نافع
سمعت رسول الله ﷺ يقول البيعان بالخيار ما لم يتفرقا أو يختارا قال فكان ابن عمر إذا ابتاع بيعا وهو قاعد قام ليجب له البيع
Reference, agreed upon. Bukhari transmitted it in Buyu, chapter, "How long is the option of withdrawal permitted?" hadith 2107, and Muslim, hadith 1531, from the hadith of Yahya bin Saeed, with this wording.On this topic there are also narrations from Abu Barza [Abu Dawud, hadith 3457], Hakeem bin Hizam [to come, 1246], Abdullah bin Amr [to come, 1247], Samura [Ibn Majah, hadith 2183, and Nasai, 7/251, hadith 4482], Abu Hurairah [to come, 1248], and Abdullah bin Abbas [Ibn Hibban, hadith 1100, and Bayhaqi, 5/270, and Hakim, 2/14].