Sunan an-Nasa'i, Volume 5, Book 44, Hadith 4474 (DarusSalam) ⓘ

It was a narrated from Ayyub, from Nafi from Ibn 'Umar that the Messenger of Allah said: "The Messenger of Allah said: "The two parties to a transaction both have the choice who long as they have not separated or one of them says to the other: 'Decide!"' (Sahih )

أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا عَبْدُ الأَعْلَى، قَالَ حَدَّثَنَا سَعِيدٌ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ الْبَيِّعَانِ بِالْخِيَارِ مَا لَمْ يَفْتَرِقَا أَوْ يَقُولَ أَحَدُهُمَا لِلآخَرِ اخْتَرْ ‏" ‏ ‏.‏