It is narrated from Sayyiduna Zayd bin Khalid al-Juhani (may Allah be pleased with him) that he heard the Messenger of Allah (peace and blessings be upon him) say: "The best of witnesses is the one who gives his testimony before he is asked about it."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5164