Sunan an-Nasa'i 4480

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 ‏"‏ ‏.‏

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

Reference

Sunan an-Nasa'i 4480

In-book

Book 44, Hadith 32