Abu Dawud Book 18, Hadith Number 3600.
Narated By Uncle of Umarah ibn Khuzaymah : The Prophet (PBUH) bought a horse from a Bedouin. The Prophet (PBUH) took him with him to pay him the price of his horse. The Apostle of Allah (PBUH) walked quickly and the Bedouin walked slowly. The people stopped the Bedouin and began to bargain with him for the horse as and they did not know that the Prophet (PBUH) had bought it.
The Bedouin called the Apostle of Allah (PBUH) saying: If you want this horse, (then buy it), otherwise I shall sell it. The Prophet (PBUH) stopped when he heard the call of the Bedouin, and said: Have I not bought it from you? The Bedouin said: I swear by Allah, I have not sold it to you. The Prophet (PBUH) said: Yes, I have bought it from you. The Bedouin began to say: Bring a witness. Khuzaymah ibn Thabit then said: I bear witness that you have bought it. The Prophet (PBUH) turned to Khuzaymah and said: On what (grounds) do you bear witness?
He said: By considering you trustworthy, Apostle of Allah (PBUH)! The Prophet (PBUH) made the witness of Khuzaymah equivalent to the witness of two people.