What is the virtue of guarding/standing watch (Haras) in the cause of Allah (fi sabilillah)?

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Guarding in the cause of Allah carries immense virtue. Two eyes are protected from the Fire: the eye that weeps out of fear of Allah, and the eye that stays awake guarding in the cause of Allah. This is supported by a Hadith narrated by Ibn Abbas, classified as Hasan Gharib by Al-Tirmidhi. Furthermore, a general supplication for mercy was made for the guard: "May Allah have mercy upon the guard of the guard." Guarding for one night in the cause of Allah is superior to a thousand nights of standing in prayer during its night and fasting during its day, as narrated by Uthman. This is supported by the account of Sahal ibn al-Hanzaliyyah regarding the Prophet's instructions to the guard Anas ibn Abi Murthad Al-Ghanawi during the Battle of Hunayn, whose diligence led the Prophet to declare him justified ('Awjabat').