It is narrated from Sayyiduna Zayd 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 inform you of the best of witnesses? They are those who come forward with their testimony before they are asked for it."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5166