Sunan an-Nasa'i, Book 44, Hadith 32 (In-Book) ⓘ
It was narrated from Sufyan, from 'Abdullah bin Dinar, from Ibn 'Umar, from the Prophet who said: "Two traders have the choice as long as they have not separated, or, they have chosen to conclude the transaction."
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " الْبَيِّعَانِ بِالْخِيَارِ مَا لَمْ يَتَفَرَّقَا أَوْ يَكُونَ بَيْعُهُمَا عَنْ خِيَارٍ " .
also referenced under: Hadith 4480 (Reference) & Volume 5, Book 44, Hadith 4485 (DarusSalam)