The Bedouin called the Messenger of Allah (ﷺ) saying: If you want this horse, (then buy it), otherwise I shall sell it. The Prophet (ﷺ) 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 (ﷺ) 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 (ﷺ) turned to Khuzaymah and said: On what (grounds) do you bear witness?
He said: By considering you trustworthy, Messenger of Allah (ﷺ)! The Prophet (ﷺ) made the witness of Khuzaymah equivalent to the witness of two people.
In-book : Book 25, Hadith 37
English translation : Book 24, Hadith 3600