Musnad Ahmad 30:3005

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