It is narrated from Sayyidah Umm al-Darda' (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said: "Whoever stands guard at any of the coasts of the Muslims for three days, it suffices him for a year of standing guard."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21018