Shall I not inform you who the best witness is? He is the one who gives his testimony before he is asked for it. [Al-Bukhari]
«ألا أُخبِرُكُم بخير الشُّهَدَاء الذي يَأتي بِشَهادَتِهِ قبل أن يُسْأَلَهَا». [صحيح.] - [رواه مسلم.]