{ Say (O Muhammad): “If the Most Gracious (Allaah) had a son, then I am the first of worshippers } [Az Zukhruf: 81]
There is also a possibility that the meaning of the Ayah is: If the Most Merciful has a son, then I am the first person to worship Allaah, and included in my worship to Allaah, is affirming what He affirmed for Himself and negating what He negated for Himself, and this worship is of creed and speech. This necessitates that if Allaah truly had a son, I would be the first to establish that for him.
So the error and falseness of the claim of the polytheists becomes evident by this, intellectually as well as textually.
{ Glorified is the Lord of the heavens and the earth, the Lord of the Throne! Exalted is He from all that they ascribe. } [Az Zukhruf: 82]
Including having a partner, an assistant, a son and other than that from what the polytheists have attribute to him. Knowing Allah.com