Musnad Ahmad 456:34

Isma'il narrated to us, he said: 'Abd al-Rahman ibn Ishaq narrated to us, from Muhammad ibn Abi Bakr ibn Hazm, from his father, from 'Abd al-Rahman ibn 'Amr ibn 'Uthman, from Zayd ibn Khalid al-Juhani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of testimony is that which the witness gives before he is asked for it."