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 tell you of the best of witnesses? It is the one who comes forward with his testimony before he is asked for it," or (he said): "who informs of his testimony before he is asked for it."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5163