Musnad Ahmad 456:12

Ishaq ibn Isa narrated to us, Malik informed us, from Abdullah ibn Abi Bakr, from his father, from Abdullah ibn Amr ibn Uthman, from Abu Amrah al-Ansari, from Zayd ibn Khalid al-Juhani—Ishaq said: "If Allah wills"—he said: The Prophet, peace and blessings of Allah be upon him, said: "Shall I not inform you of the best of witnesses? The one who brings the testimony before he is asked for it."