Sunan an-Nasā'ī 4466✓ 5Sahih
أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، قَالَ حَدَّثَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " الْبَيِّعَانِ بِالْخِيَارِ مَا لَمْ يَفْتَرِقَا أَوْ يَكُونَ خِيَارًا " .
It was narrated from Yahaya, from 'Ubaidullah who said:"Nafi narrated to me from Ibn 'Umar, tht the Messenger of Allah said: 'the two parties to a transaction both have the choice so long as they have not separated, or they have chosen."