Musnad Ahmad 30:2410

Yahya narrated to us, he said: Al-Muthanna ibn Sa'id informed us, he said: Qatadah narrated to us, from Bashir ibn Ka'b, from Abu Hurairah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "If you differ, or dispute regarding the road, then leave seven cubits."