Musannaf Ibn Abi Shayba 33:1196

Waki' and Yazid ibn Harun narrated to us, from Isma'il ibn Abi Khalid, from Yahya ibn Rafi', who said: I heard 'Uthman say: "And every soul shall come, with it a driver and a witness." He said: A driver who drives it to the command of Allah, and a witness who testifies against it regarding what it did.