جامع الترمذي

Narrated Muhammad bin Al-Munkadir:

"Salman Al-Farisi passed by Shurahbil bin As-Simt while he was in garrison in which he and his companions were suffering from difficulties. He said to him: 'Shall I narrate to you - O Ibn As-Simt - a Hadith I heard from the Messenger of Allah (ﷺ) ?' He said: 'Of course.' He said: 'I heard the Messenger of Allah (ﷺ) saying: "(Ribat) Guarding the frontier for a day in the cause of Allah is more virtuous" - and perhaps he said: "better, than fasting a month and standing (in prayer) for it. And whoever dies in it, he is protected from the trials of the grave, and his deeds (continuously) multiplied until the Day of Resurrection.

[Abu 'Eisa said:] This Hadith is Hasan.