Tafsir of Ash-Shu`ara' 26:16

The fa (so) in the words of the Exalted, "So go both of you to Pharaoh and say: We are the messengers of the Lord of the worlds," is for ordering what follows it upon what preceded it of the noble promise. This is not merely an emphasis on the command to go, because its meaning is the arrival at the one being approached, not merely setting out towards the one being approached, as is [the case with] al-dhahab (going).

Al-rasul (messenger) is made singular here because it is a verbal noun (masdar) according to the root, which is used as a descriptor just as other verbal nouns are used for the sake of hyperbole, like [the phrase] rajul 'adl (a just man). It thus undergoes the same grammatical considerations as the latter, and the [relevant] considerations are not hidden. Among those supporting its status as a verbal noun is the obvious statement of Kathir 'Azza: "The slanderers have lied; I have never spoken a secret to them, nor have I sent them as a messenger (rasulan)." Even clearer than that is the statement of al-'Abbas ibn Mirdas: "Will no one convey from me to Khaffaf a messenger (rasulan)..."

[It is singular] because of the unity of the brothers, or the unity of the Sender, or [the unity of] what is being sent with, or because His saying, "Inna" (We are), means "our speech is," so it is correct to make the predicate singular just as it is correct in that [instance]. Its benefit is to indicate that each of them is commanded to convey that, even if individually. In the expression "Lord of the worlds" is a refutation of the accursed one and an overturning of what he had established of his claim to divinity, and a subtle urging for him to comply with the command.