It is narrated from Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) that he passed by the Prophet (peace and blessings be upon him) when he (peace and blessings be upon him) had bought a horse from a Bedouin, but the Bedouin flatly denied the sale, saying: "I did not sell this horse to you." The Prophet (peace and blessings be upon him) said: "Indeed you did; you sold it to me." Meanwhile Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) passed by there and heard their exchange, and he exclaimed: "I bear witness that you have bought it." The Prophet (peace and blessings be upon him) asked him: "How do you know about this matter when you were not present with us at the time of the deal?" He said: "We have borne witness to something far greater and more momentous than this (worldly matter), (namely divine revelation and your prophethood, so how could we doubt your truthfulness in this matter?)" So the Prophet (peace and blessings be upon him) made his single testimony, in the Shari'ah, equal to the testimony of two men, and this honour remained with Sayyiduna Khuzaymah (may Allah be pleased with him) until his death.
Likewise, it is narrated through another chain from Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) bought a horse from Sawa' bin Qays al-Muharibi, who later denied it, so Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) bore witness in his (peace and blessings be upon him) favour. The Messenger of Allah (peace and blessings be upon him) asked him: "What moved you to give this testimony when you were not present on the spot?" He said: "I have affirmed the religion and law you have brought, and I know well that nothing but the truth comes from your blessed tongue." Upon this, the Messenger of Allah (peace and blessings be upon him) declared this virtue: "Whoever Khuzaymah bears witness for or against, that suffices him."
Imam Ibn Abi Umar, Abu Bakr bin Abi Shaybah and Abu Ya'la narrated this hadith with their respective chains; its origin is also found in Sunan Abi Dawud and al-Nasa'i, and it is sahih (authentic).
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب المناقب / 4019
Hadith Gradingمحدثین:إسناده حسن صحيح
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :هَذَا الحَدِيثُ صَحِيحٌ ثَابِتٌ بَالِغُ الصِّحَّةِ؛ فَالطَّرِيقُ الأَوَّلُ (عِنْدَ ابْنِ أَبِي عُمَرَ) إِسْنَادُهُ صَحِيحٌ عَلَى شَرْتِ الشَّيْخَيْنِ، وَالطَّرِيقَانِ الثَّانِي وَالثَّالِثُ (عِنْدَ أَبِي بَكْرٍ وَأَبِي يَعْلَى) إِسْنَادُهُمَا حَسَنٌ. وَأَصْلُ القِصَّةِ مَشْهُورٌ ثَابِتٌ فِي "سُنَنِ أَبِي دَاوُدَ" (برقم: 3607) » «أخرجه المستدرك على الصحيحين برقم: 2234، وأخرجه المعجم الكبير للطبراني برقم: 3730، وأخرجه الأوسط في السنن والإجماع والاختلاف برقم: 6752، وأخرجه إتحاف الخيرة المهرة بزوائد المسانيد العشرة برقم: 6835/2»