Musnad Ahmad 27:1436

Ishaq ibn Isa and Aswad ibn Amir narrated to us, they said: Sharik narrated to us, from Yazid ibn Abi Ziyad, from Abd al-Rahman ibn Abi Layla, from Ibn Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent us in a military expedition. When we met the enemy, we fled during the first charge. We arrived in Medina among a group at night and hid ourselves. Then we said: "If only we went out to the Messenger of Allah (may Allah bless him and grant him peace) and apologized to him." So we went out, and when we met him, we said: "We are the ones who fled, O Messenger of Allah." He said: "Rather, you are the ones who return to the fight (al-akkarun), and I am your group (fi'ah)." Aswad ibn Amir said: "And I am the group of every Muslim."