Sunan an-Nasai 19227
In-book no 4491أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا مُعَاذُ بْنُ هِشَامٍ، قَالَ حَدَّثَنِي أَبِي، عَنْ قَتَادَةَ، عَنِ الْحَسَنِ، عَنْ سَمُرَةَ، أَنَّ نَبِيَّ اللَّهِ صلى الله عليه وسلم قَالَ " الْبَيِّعَانِ بِالْخِيَارِ حَتَّى يَتَفَرَّقَا أَوْ يَأْخُذَ كُلُّ وَاحِدٍ مِنْهُمَا مِنَ الْبَيْعِ مَا هَوِيَ وَيَتَخَايَرَانِ ثَلاَثَ مَرَّاتٍ " .
It was narrated from Al_Hasan, from Samurah, that the Prophet of Allah said: "Two trades have the choice as long as until they reach a deal that suits both of them and they confirm it three times. "