Sunan an-Nasai 19215
In-book no 4479أَخْبَرَنَا 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 " .
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 )