Zaid bin Khalid Al-Juhani narrated that the Messenger of Allah(s.a.w) said:
"The best of witnesses is the one who gives his testimony before being asked for it." (JA)
English reference (JA): Vol. 4, Book 9, Hadith 2297 (JA)
Arabic reference (JA): Book 35, Hadith 2466 (JA)