Say "If the Most Gracious (Allah) had a son, then I am the first of worshippers to that son, because he would only be a part of his father, and I am the first of the creation to implement what Allah loves. However I am the first one to rebuke it, and the most stern in negating it.">
Say (O Muhammad): "If the Most Gracious (Allah) had a son, then I am the first of worshippers
[Az Zukhruf: 81]
Meaning say, O noble Messenger, to those who claim that Allah has a son, while He is the One, the Only, the Single, the Self sufficient, One who did not take a female companion nor a son and there is no like unto Him. Say "If the Most Gracious (Allah) had a son, then I am the first of worshippers to that son, because he would only be a part of his father, and I am the first of the creation to implement what Allah loves. However I am the first one to rebuke it, and the most stern in negating it.
So its falseness is known by this, and this is a great evidence for those who know the state of the Messenger, since it is known that he is the most complete person, and that he is the first to practice and perfect any form of good, and the first to forsake, rebuke and distance himself from all forms of evil. Since this was his state, if the Most Merciful had a son, Muhammad ibn ‘Abdullah - صلى الله عليه و سلم, the most virtuous of the Messengers would have been the first to worship him, and the polytheists would not have beaten him to it.
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 Allah, and included in my worship to Allah, 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 Allah 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.
أي: قل يا أيها الرسول الكريم، للذين جعلوا للّه ولدا، وهو الواحد الأحد الفرد الصمد، الذي لم يتخذ صاحبة ولا ولدا، ولم يكن له كفوا أحد. قُلْ إِنْ كَانَ لِلرَّحْمَنِ وَلَدٌ فَأَنَا أَوَّلُ الْعَابِدِينَ لذلك الولد، لأنه جزء من والده، وأنا أول الخلق انقيادا للأمور المحبوبة للّه، ولكني أول المنكرين لذلك، وأشدهم له نفيا، فعلم بذلك بطلانه، فهذا احتجاج عظيم عند من عرف أحوال الرسل، وأنه إذا علم أنهم أكمل الخلق، وأن كل خير فهم أول الناس سبقا إليه وتكميلا له، وكل شر فهم أول الناس تركا له وإنكارا له وبعدا منه، فلو كان على هذا للرحمن ولد وهو الحق، لكان محمد بن عبد اللّه، أفضل الرسل أول من عبده، ولم يسبقه إليه المشركون.
ويحتمل أن معنى الآية: لو كان للرحمن ولد، فأنا أول العابدين للّه، ومن عبادتي للّه، إثبات ما أثبته، ونفي ما نفاه، فهذا من العبادة القولية الاعتقادية، ويلزم من هذا، لو كان حقا، لكنت أول مثبت له، فعلم بذلك بطلان دعوى المشركين وفسادها، عقلا ونقلا.