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Shiavault - a Vault of Shia Islamic Books Al-Muraja'at Letter 42 Muharram 4, 1330 I Arabs Address the Singular Using the Plural Form, The answer to your question is that Arabs apply the plural expression while addressing an individual due to the nice effect it produces [i.e. respect].
II Testimonials, A testimony to this fact is what the Almighty says in Surat Al-i-’Imran: Those to whom some people said: "A large army has been raised against you; so, fear them," yet it only increased their faith, and they said: "Allah suffices us, and He is the One upon Whom we depend most." (Qur'an, 3:173) The person implied in these verses of Al-i-’Imran is none other than Na’im ibn Mas’ud al-Ashja’i, according to the consensus of scholars of exegesis, traditionists, and chroniclers.
Yet Allah Almighty has applied to him, the singular person that he is, the plural form just to express respect for those who did not listen to his statements nor heeded his dissuading calls. Abu Sufyan had given him ten camels in order to demoralize and frighten the Muslims regarding the strength of the polytheists, and he did just that.
Among his statements then was: "People have gathered a mighty force to attack you; so, fear for your own lives." Many Muslims disliked the idea of fighting that force just because of his statement, but the Messenger of Allah, peace be upon him and his progeny, came out accompanied by seventy cavaliers to meet them, and they all returned from the battle-field safely, whereupon this verse was revealed praising the seventy believers who came out with the Messenger of Allah, peace be upon him and his progeny, heedless to the dissuasion of those who wished to demoralize them.
In applying the word "people" for just one individual, a nice and divine point is made which is complimenting the seventy men who came out with the Prophet. This surely sounds more eloquent when used as such; it is better than saying: "Those to whom a man said that a large army had been raised..., etc.," as is obvious. There are nUmarous verses in the Holy Qur'an similar to this one, as well as in the Arabic language as a whole. The Almighty Allah says: "O you who believe!