Sunan an-Nasa'i 4647
It was narrated from 'Umarah bin Khuzaimah that his paternal uncle, who was one of the companions of the Prophet told him, that:
the Prophet bought a horse from a Bedouin and asked him to follow him, so that he could pay him for the horse. The Prophet hastened but the Bedouin was slow. Men started to talk to the Bedouin and make offers for the horse, and they did not realize that the Prophet had bought it, until some of them offered more than the Prophet had bought it for. Then the Bedouin called out to the Prophet and said; "Are you going to buy this horse or shall I sell it?" The Prophet stood up when he heard him calling and said: "Have I not bought it from you?" He said: 'No, by Allah, I have not sold it to you, and the Prophet said "I bought it from you." The people started to gaiter around the Prophet and the Bedouion as they were talking, and the Bedouin started to say: "Bring a witness who will testify that you bought it. " Khuzaimah bin habit said: "I bear witness that you bought it" The Prophet turned to Khunzimah and said: "Why are you bearing witness?" He said: "Because I know that you are truthful, O Messenger of Allah" made the testimony of Khuzaimah equivalent to the testimony of two men. (sahih)
أَخْبَرَنَا الْهَيْثَمُ بْنُ مَرْوَانَ بْنِ الْهَيْثَمِ بْنِ عِمْرَانَ، قَالَ حَدَّثَنَا مُحَمَّدُ بْنُ بَكَّارٍ، قَالَ حَدَّثَنَا يَحْيَى، - وَهُوَ ابْنُ حَمْزَةَ - عَنِ الزُّبَيْدِيِّ، أَنَّ الزُّهْرِيَّ، أَخْبَرَهُ عَنْ عُمَارَةَ بْنِ خُزَيْمَةَ، أَنَّ عَمَّهُ، حَدَّثَهُ - وَهُوَ، مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم - أَنَّ النَّبِيَّ صلى الله عليه وسلم ابْتَاعَ فَرَسًا مِنْ أَعْرَابِيٍّ وَاسْتَتْبَعَهُ لِيَقْبِضَ ثَمَنَ فَرَسِهِ فَأَسْرَعَ النَّبِيُّ صلى الله عليه وسلم وَأَبْطَأَ الأَعْرَابِيُّ وَطَفِقَ الرِّجَالُ يَتَعَرَّضُونَ لِلأَعْرَابِيِّ فَيَسُومُونَهُ بِالْفَرَسِ وَهُمْ لاَ يَشْعُرُونَ أَنَّ النَّبِيَّ صلى الله عليه وسلم ابْتَاعَهُ حَتَّى زَادَ بَعْضُهُمْ فِي السَّوْمِ عَلَى مَا ابْتَاعَهُ بِهِ مِنْهُ فَنَادَى الأَعْرَابِيُّ النَّبِيَّ صلى الله عليه وسلم فَقَالَ إِنْ كُنْتَ مُبْتَاعًا هَذَا الْفَرَسَ وَإِلاَّ بِعْتُهُ ‏.‏ فَقَامَ النَّبِيُّ صلى الله عليه وسلم حِينَ سَمِعَ نِدَاءَهُ فَقَالَ ‏"‏ أَلَيْسَ قَدِ ابْتَعْتُهُ مِنْكَ ‏"‏ ‏.‏ قَالَ لاَ وَاللَّهِ مَا بِعْتُكَهُ ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ قَدِ ابْتَعْتُهُ مِنْكَ ‏"‏ ‏.‏ فَطَفِقَ النَّاسُ يَلُوذُونَ بِالنَّبِيِّ صلى الله عليه وسلم وَبِالأَعْرَابِيِّ وَهُمَا يَتَرَاجَعَانِ وَطَفِقَ الأَعْرَابِيُّ يَقُولُ هَلُمَّ شَاهِدًا يَشْهَدُ أَنِّي قَدْ بِعْتُكَهُ ‏.‏ قَالَ خُزَيْمَةُ بْنُ ثَابِتٍ أَنَا أَشْهَدُ أَنَّكَ قَدْ بِعْتَهُ ‏.‏ قَالَ فَأَقْبَلَ النَّبِيُّ صلى الله عليه وسلم عَلَى خُزَيْمَةَ فَقَالَ ‏"‏ لِمَ تَشْهَدُ ‏"‏ ‏.‏ قَالَ بِتَصْدِيقِكَ يَا رَسُولَ اللَّهِ ‏.‏ قَالَ فَجَعَلَ رَسُولُ اللَّهِ صلى الله عليه وسلم شَهَادَةَ خُزَيْمَةَ شَهَادَةَ رَجُلَيْنِ ‏.‏