"Every deceased person has his deeds sealed, except for one who dies guarding the frontiers (murābit) in the way of Allah. For his deeds will continue to grow for him until the Day of Resurrection, and he will be safe from the trial of the grave."

Reported by Aḥmad (no. 23951), Abū Dāwūd (no. 2500), and al-Tirmidhī (no. 1621, with his wording), from the narration of Faḍālah ibn ʿUbayd — may Allah be pleased with him.
Authenticated in Ṣaḥīḥ al-Jāmiʿ (no. 4562) and Ṣaḥīḥ al-Targhīb wa al-Tarhīb (no. 1218).

Brief Explanation of the Hadith

Guarding the frontiers in the path of Allah is among the greatest deeds and the most richly rewarded, for it serves to elevate the word of Allah and spread His religion. Among the proofs of its immense virtue is what the Prophet — may Allah’s peace and blessings be upon him — informed: when a servant dies, his deeds come to an end and his record is sealed, with no increase or decrease in reward, except for the one who dies while stationed in ribāṭ in the path of Allah. For his deeds that he used to perform in this world continue to run for him, and beyond that, his reward is increased, and his actions are multiplied for him until the Day of Resurrection. Thus, he (the murābit) receives two rewards: the reward for the continuation of his deeds upon which he died, and the reward for the multiplication of his recompense. This is due to the protection of the sanctity of the religion inherent in guarding the frontiers for Allah Almighty's sake, and its widespread benefit to Muslims.
Moreover, the one stationed in ribāṭ is spared from the trial of the grave; the two angels do not question him as they question the rest of the people of the graves. Rather, his death in the path of Allah suffices as a testimony to the soundness of his faith. This is a manifest virtue for the murābiṭ, by which he is distinguished from others—his deeds continuing after his death, coupled with their growth—indicating the unique status of this deed and the nobility of its rank.