Musnad Ahmad 529:4

'Isam ibn Khalid narrated to us, Abu 'Abd Allah al-Hasan ibn Ayyub narrated to us, 'Abd Allah ibn Nasih al-Hadrami narrated to me, he said: 'Utbah ibn 'Abd narrated to me, he said: The Messenger of Allah, peace and blessings of Allah be upon him, commanded [the people] to fight. A man from his companions was struck by an arrow, so the Messenger of Allah, peace and blessings of Allah be upon him, said: "This one has made [Paradise] obligatory." And they said when he commanded them to fight: "Then, O Messenger of Allah, we will not say as the Children of Israel said: 'Go, you and your Lord, and fight; indeed, we are sitting right here.' Rather, go, you and your Lord, and fight; indeed, we are with you, fighting."