Musnad Ahmad 939:1

Isma'il ibn Ibrahim narrated to us, 'Abd al-Rahman ibn Ishaq informed us, from Abu Bakr ibn Muhammad ibn 'Amr ibn Hazm, from his father, from 'Abd Allah 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 one who gives it bears before he is asked for it."