Zaid bin Khalid Al-Juhani narrated that the Messenger of Allah (s.a.w) said:
"Shall l not inform you of the best of witnesses? The one who comes with his testimony before being asked for it." (JA)
English reference (JA): Vol. 4, Book 9, Hadith 2295 (JA)
Arabic reference (JA): Book 35, Hadith 2464 (JA)