It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that I heard the Prophet (peace and blessings of Allah be upon him) say: "If the seller and the buyer have a dispute and neither of them has a witness, then the statement of the seller will be accepted; otherwise, both should return their respective items."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4445
Hadith Gradingحکم دارالسلام:حسن، وهذا إسناد ضعيف لانقطاعه ، القاسم لم يدرك جده، عبدالرحمن بن عبد الله المسعودي أختلط ، وسماع وكيع منه قبل الاختلاط.