Musnad Ahmad 939:14

Zayd ibn al-Hubab narrated to us, Ubayy ibn Abbas ibn Sahl ibn Sa'd al-Sa'idi narrated to me, Abu Bakr ibn Muhammad ibn 'Amr ibn Hazm narrated to me, 'Abdullah ibn 'Amr ibn 'Uthman ibn 'Affan narrated to me, Kharijah ibn Zayd ibn Thabit al-Ansari narrated to me, 'Abd al-Rahman ibn Abi 'Amrah al-Ansari narrated to me, Zayd ibn Khalid al-Juhani narrated to me, that he heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "The best of witnesses is the one who performs his testimony before he is asked for it."