It is narrated from Sayyiduna Mu'adh bin Anas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever stands guard behind the Muslims (to protect them) in the path of Allah voluntarily, with no ruler compelling him to do so, will not see the Fire with his eyes except to fulfil the oath (that is, only to the extent of crossing the Sirat bridge), for Allah Almighty says: 'There is none of you except that he will come to it.' [Surah Maryam: 71]"
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17537