Musnad Abu Ya'la al-Mawsili 118:36

Mu'ammal ibn Ihab narrated to us, Abd al-Razzaq narrated to us, from Ma'mar, from al-Zuhri, from Anas, that the Prophet (peace be upon him) entered during the Umrat al-Qada (the compensatory Umrah), and Ibn Rawahah was in front of him, saying: "Clear the way, O sons of the disbelievers, from his path. The Most Merciful has revealed in His revelation that the best of killing is in His path."