Musnad Abu Ya'la al-Mawsili 151:18

Abu Ma'mar Isma'il al-Hudhali narrated to us, Hushaym narrated to us, Ibn Abi Layla informed us, from al-Qasim ibn 'Abd al-Rahman, from his father, from 'Abdullah ibn Mas'ud: That al-Ash'ath ibn Qays bought some slaves from the slaves of the emirate, and they disagreed regarding the price. Ibn Mas'ud said, "I sold them to you for twenty thousand." Al-Ash'ath said, "I only bought them from you for ten thousand." 'Abdullah said, "If you wish, I will narrate to you a hadith I heard from the Messenger of Allah." He said, "Bring it." He said, "I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'If the two parties to a sale disagree and there is no evidence between them, then the word is the word of the seller, or they rescind the sale.'" He said, "Then I rescind the sale."