Jami` at-Tirmidhi 1245
جامع الترمذي#1245SahihNarrated Nafi': 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Bashar Awad MaaroufHasan Sahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 14:45