Zayd ibn Khālid al-Juhani reported that the Prophet (may Allah's peace and blessings be upon him) said: "Shall I not inform you who the best witness is? He is the one who gives his testimony before he is asked for it."
This Hadīth illustrates the merit of the one who gives a testimony before he is even asked to do so. This indicates his good nature and keenness to give his testimony in order to preserve people's rights. The Hadīth is interpreted to apply to the case when the person who has a right does not know of this testimony or forgot about it. The witness is thus recommended to take the initiative and testify. That is how this Hadīth is better understood in light of other reports that dispraise the person who gives testimony before he is asked for it.