Bulugh al-Maram, Book 14, Hadith 18 (In-Book)

Narrated Zaid bin Khaled al-Juhani (RA): The Prophet said: "Shall I not inform you who is the best witness? He is the one who produces his testimony before he is asked for it." [Reported by Muslim].

عَنْ زَيْدِ بْنِ خَالِدٍ اَلْجُهَنِيِّ ‏- رضى الله عنه ‏- أَنَّ اَلنَّبِيَّ ‏- صلى الله عليه وسلم ‏-قَالَ: { "أَلَا أُخْبِرُكُمْ بِخَيْرِ اَلشُّهَدَاءِ? اَلَّذِي يَأْتِي بِشَهَادَتِهِ قَبْلَ أَنْ يُسْأَلَهَا" } رَوَاهُ مُسْلِم ٌ 1‏.‏ ‏1 ‏- صحيح.‏ رواه مسلم ( 1719 )‏.‏

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also referenced under: Book 14, Hadith 1414 (English)