Musnad Abu Ya'la al-Mawsili 118:44

Abu Bakr ibn Zanjawayh narrated to us, Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, who informed me that Anas ibn Malik said: The Messenger of Allah (peace and blessings of Allah be upon him) entered Makkah during the Umrat al-Qada (the compensatory pilgrimage), and Ibn Rawahah was holding his stirrup while saying: "Clear the way, sons of the disbelievers, from his path. The Most Merciful has revealed in His revelation that the best of killing is in His path."