Sayyiduna Zayd bin Khalid al-Juhani (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Shall I not tell you of the best of witnesses? Those who give their testimony before they are asked."
(b) This refers to the person who holds a testimony while the person (concerned) does not know of it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20594