Musnad Ahmad 1178:3

Ishaq ibn Isa narrated to us, he said: Ismail ibn Ayyash narrated to us, from Muhammad ibn Amr ibn Halhalah al-Du'ali, from Ishaq ibn Abdullah, from Umm al-Darda, who attributed the hadith [to the Prophet], she said: "Whoever performs ribat (guarding the frontier) in any of the coastal regions of the Muslims for three days, it shall suffice him for the ribat of a year."