Shurahbil bin al-Simt narrated that he saw Sayyiduna Salman al-Farisi (may Allah be pleased with him) standing guard on the coast of Hims, and he asked: Why are you here? He said: I am standing guard. Sayyiduna Salman (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Standing guard for one day in the path of Allah is like fasting a month and standing it in prayer. Whoever dies while standing guard, the deed he used to do continues for him, he is kept safe from the trial, and on the Day of Resurrection he will be raised as a martyr."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6154