It is narrated from Sayyiduna Zaid bin Khalid Al-Juhani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Shall I not tell you about the best witness? It is the one who gives testimony on his own before being asked."
Hadith Referenceالسنن المأثورة / باب إطعام الخادم مما يأكل منه مالكه / 521
Hadith Takhrijصحیح مسلم، الأقضية، باب بیان خیر الشھود رقم: 1719