Musnad Ahmad 132:4

Hassan narrated to us, Ibn Lahi'ah narrated to us, Zaban narrated to us, and Yahya ibn Ghaylan narrated to us, Rashdin narrated to us, from Zaban, from Sahl ibn Mu'adh, from his father, from the Messenger of Allah, may Allah bless him and grant him peace, that he said: "Whoever guards behind the Muslims in the way of Allah, Blessed and Exalted is He, as a volunteer, not taken by a ruler, will not see the Fire with his eyes, except for the fulfillment of the oath, for Allah, Blessed and Exalted is He, says: 'And there is none of you except he will come to it' [Maryam: 71]."