Musnad Ahmad 944:34

Abu al-Yaman narrated to us, Shu’ayb narrated to us, from al-Zuhri, ‘Umarah ibn Khuzaymah al-Ansari narrated to me, that his paternal uncle—who was among the companions of the Prophet (peace and blessings of Allah be upon him)—narrated to him: That the Prophet (peace and blessings of Allah be upon him) bought a horse from a Bedouin. The Prophet (peace and blessings of Allah be upon him) asked the Bedouin to follow him so he could pay him the price of his horse. The Prophet (peace and blessings of Allah be upon him) walked quickly, while the Bedouin walked slowly. Men began to intercept the Bedouin and bargain with him for the horse, not realizing that the Prophet (peace and blessings of Allah be upon him) had bought it, until some of them offered the Bedouin more than the price for which the Prophet (peace and blessings of Allah be upon him) had bought it. The Bedouin called out to the Prophet (peace and blessings of Allah be upon him) and said: "If you are going to buy this horse, then buy it; otherwise, I will sell it." The Prophet (peace and blessings of Allah be upon him) stopped when he heard the Bedouin’s call and said: "Have I not already bought it from you?" The Bedouin said: "No, by Allah, I did not sell it to you." The Prophet (peace and blessings of Allah be upon him) said: "Yes, I have indeed bought it from you." People began to gather around the Prophet (peace and blessings of Allah be upon him) and the Bedouin as they debated. The Bedouin began to say: "Bring a witness who will testify that I sold it to you." Every Muslim who came by said to the Bedouin: "Woe to you! The Prophet (peace and blessings of Allah be upon him) would not say anything but the truth." Until Khuzaymah came while the Prophet (peace and blessings of Allah be upon him) and the Bedouin were debating, and the Bedouin was saying: "Bring a witness who will testify that I sold it to you." Khuzaymah said: "I testify that you have sold it to him." The Prophet (peace and blessings of Allah be upon him) turned to Khuzaymah and said: "On what basis do you testify?" He replied: "By believing you, O Messenger of Allah." The Prophet (peace and blessings of Allah be upon him) then made the testimony of Khuzaymah equal to the testimony of two men.