It is narrated from Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) bought a horse from Sawa' bin al-Harith, and then he denied it, so Sayyiduna Khuzaymah (may Allah be pleased with him) testified in his (peace and blessings be upon him) favour. The Messenger of Allah (peace and blessings be upon him) said to him: "What prompted you to testify when you were not even present there?" He said: I have affirmed the truth of what you have brought (the religion), and I know that you say nothing but the truth. So the Messenger of Allah (peace and blessings be upon him) said: "Whoever Khuzaymah testifies for or against, that is sufficient for him."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3715