Musannaf Ibn Abi Shayba 13:3075

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Sufyan narrated to us, from Simak, from Ikrimah, from Ibn Abbas, who said: The Prophet (peace and blessings of Allah be upon him) said: "If you differ regarding the road, then make it seven cubits."