Musannaf Ibn Abi Shayba 33:423

Waki’ narrated from Abu Mawdud, from Nafi’, from Ibn ‘Umar, that he used to say while on the road to Makkah, at the head of his riding beast, turning it: "Perhaps a hoof may fall upon a hoof," meaning the hoof of the riding beast of the Prophet, peace and blessings of Allah be upon him.