Musnad Ahmad 482:144

Hassan, Abu Sa’id, and Yahya ibn Ishaq narrated to us, they said: Ibn Lahi’ah narrated to us, Mushrah ibn Ha’an narrated to us, from ‘Uqbah ibn ‘Amir—Yahya ibn Ishaq said—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—Yahya said: in the way of Allah—for indeed, the reward for his deeds continues to flow for him until Allah resurrects him."