Narrated Ibn Mas'ud (RA):

I heard Allah's Messenger (ﷺ) saying, "When two people who are arranging a business transaction disagree and there is no proof to arbitrate between them, the seller's word is final, or they may break the deal." [Reported by al-Khamsa and authenticated by al-Hakim].

وَعَنْ اِبْنِ مَسْعُودٍ ‏- رضى الله عنه ‏- قَالَ: سَمِعْتُ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-يَقُولُ: { إِذَا اِخْتَلَفَ اَلْمُتَبَايِعَانِ لَيْسَ [1]‏ بَيْنَهُمَا بَيِّنَةٌ, فَالْقَوْلُ مَا يَقُولُ رَبُّ اَلسِّلْعَةِ أَوْ يَتَتَارَكَانِ } رَوَاهُ اَلْخَمْسَةُ, وَصَحَّحَهُ اَلْحَاكِمُ [2]‏ .‏