Musnad Ahmad 482:68

Abdullah ibn Yazid narrated to us, Ibn Lahi'ah narrated to us, Musharrah narrated to us, he said: I heard Uqbah ibn Amir saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Every dead person has his deeds sealed, except for the one who guards the frontier in the way of Allah, for his deeds continue to be credited for him until he is resurrected." Qutaybah narrated to us, he added in it: "And he is secured from the trial of the grave."