It is narrated from Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Everyone who dies has his deeds sealed, except the one who dies guarding the frontier (murabit) in the path of Allah, for his deeds continue to grow for him, and he is kept safe from the trial of the grave."
Hadith Referenceالمعجم الكبير للطبراني / باب الفاء / 15893