Umarah bin Khuzaymah (may Allah have mercy on him) narrates that his paternal uncle, who was a Companion of the Prophet (peace and blessings be upon him), told him that the Prophet (peace and blessings be upon him) bought a horse from a Bedouin, and he (peace and blessings be upon him) walked ahead of him to give him the price of his horse. The Prophet (peace and blessings be upon him) walked quickly and the Bedouin lagged behind, so some men began to approach the Bedouin and bargain with him over the horse, not knowing that the Prophet (peace and blessings be upon him) had already bought it, until one of them offered the Bedouin more than the price at which the Prophet (peace and blessings be upon him) had bought the horse. So the Bedouin called out to the Prophet (peace and blessings be upon him) and said: If you are going to buy this horse, then buy it; otherwise I will sell it. When the Prophet (peace and blessings be upon him) heard the Bedouin's call, he said: "Have I not already bought it from you?" He said: No, by Allah, we did not make a sale with you. The Prophet (peace and blessings be upon him) said: "Indeed, I have bought it from you." The people began to gather around the Prophet (peace and blessings be upon him) and the Bedouin while the two of them were arguing back and forth, and the Bedouin began to say: Bring a witness who will testify that I sold it to you. Every Muslim who came said to the Bedouin: Woe to you! The Prophet (peace and blessings be upon him) says nothing but the truth. Then Sayyiduna Khuzaymah (may Allah be pleased with him) came and listened to the exchange between the Prophet (peace and blessings be upon him) and the Bedouin, and the Bedouin began to say: Bring a witness that I sold it to you. Sayyiduna Khuzaymah (may Allah be pleased with him) said: I bear witness that you sold it to him. The Prophet (peace and blessings be upon him) turned to Sayyiduna Khuzaymah (may Allah be pleased with him) and said: "On what basis do you testify?" He said: O Messenger of Allah, on the basis of my belief in you. So the Messenger of Allah (peace and blessings be upon him) made the testimony of Sayyiduna Khuzaymah (may Allah be pleased with him) equal to the testimony of two men.
Hadith Referenceالمعجم الكبير للطبراني / مسند من يعرف بالكنى / 19149