Musnad Ahmad 456:19

Safwan ibn Isa narrated to us, he said: Muhammad ibn Umarah informed us, from Abu Bakr ibn Muhammad, from Abdullah ibn Amr, 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 testimony? Those who bring forth their testimony without being asked for it."