In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Allaah [The Most High] says in Surah Ash-Shu’araa Aayaat 23-29:
[قَالَ فِرۡعَوۡنُ وَمَا رَبُّ ٱلۡعَـٰلَمِينَ- Fir’aun (Pharaoh) said: “And what is the Lord of the ‘Alamin (mankind, jinns and all that exists)?]; [ قَالَ رَبُّ ٱلسَّمَـٰوَٲتِ وَٱلۡأَرۡضِ وَمَا بَيۡنَهُمَآۖ إِن كُنتُم مُّوقِنِينَ – Musa (Moses) said: “Lord of the heavens and the earth, and all that is between them, if you seek to be convinced with certainty];
[قَالَ لِمَنۡ حَوۡلَهُ ۥۤ أَلَا تَسۡتَمِعُونَ – Fir’aun (Pharaoh) said to those around: “Do you not hear (what he (Moosaa) says)?]; [قَالَ رَبُّكُمۡ وَرَبُّ ءَابَآٮِٕكُمُ ٱلۡأَوَّلِينَ – Musa (Moses) said: “Your Lord and the Lord of your ancient fathers!]; [قَالَ إِنَّ رَسُولَكُمُ ٱلَّذِىٓ أُرۡسِلَ إِلَيۡكُمۡ لَمَجۡنُونٌ۬ – Fir’aun (Pharaoh) said: “Verily, your Messenger who has been sent to you is a madman!]; [قَالَ رَبُّ ٱلۡمَشۡرِقِ وَٱلۡمَغۡرِبِ وَمَا بَيۡنَہُمَآۖ إِن كُنتُمۡ تَعۡقِلُونَ – Musa (Moses) said: “Lord of the east and the west, and all that is between them, if you did but understand!]; [قَالَ لَٮِٕنِ ٱتَّخَذۡتَ إِلَـٰهًا غَيۡرِى لَأَجۡعَلَنَّكَ مِنَ ٱلۡمَسۡجُونِينَ – Fir’aun (Pharaoh) said: “If you choose an ilah (god) other than me, I will certainly put you among the prisoners].
So after Fir’awn had no proofs to offer and was completely unable to offer anything that would nullify the proofs of his opponent, he utilised his authority in the land to threaten Moosaa [alayhis- salaam]. He -May Allaah disfigure him- thought that he could misguide Moosaa [alayhis-salaam] by threatening him- that Moosaa cannot worship a god besides him, for if Moosaa [alayhis-salaam] does not choose him as his god, then indeed Moosaa has affirmed that he and his followers are upon clear-sightedness in the affairs of their religion.
[An Excerpt from ‘Tayseer Al-kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan’ by Imaam As-Sadi (rahimahullaah)]