Musnad Ahmad 939:10

Abu Nuh Qarrad narrated to us, Malik ibn Anas narrated to us, from Abdullah ibn Abi Bakr, from Abdullah ibn Amr ibn Uthman ibn Affan, from Ibn Abi Amrah, from Zayd ibn Khalid al-Juhani, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Shall I not inform you of the best of witnesses? The one who brings his testimony before he is asked for it," or "he informs of his testimony before he is asked for it."