In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Allaah [The Most High] stated in Surah Ash-Shu’araa about Fir’awn [may Allaah curse him] and Musa [peace be upon him]: [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 (Musa) 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 Musa [peace be upon him]. He -May Allaah disfigure him- thought that he could misguide Musa by threatening him, and that Musa cannot worship a god besides him, because if Musa does not choose him as his god, then indeed Musa has affirmed that he and his followers are upon clear-sightedness in the affairs of their religion.

Allaah [The Most High] informed us that Fir’awn said:

[أَمۡ أَنَا۟ خَيۡرٌ۬ مِّنۡ هَـٰذَا ٱلَّذِى هُوَ مَهِينٌ۬ وَلَا يَكَادُ يُبِينُ – Am I not better than this one [Musa (Moses)], who is Mahin (has no honour nor any respect, and is weak and despicable) and can scarcely express himself clearly?][Surah Az-Zukhruf’ Ayah 52]

May Allaah [The Most High] disfigure Fir’awn, because indeed he intended Musa by this description, even though Musa is one whom Allaah [The Most High] spoke to directly and one given high status by Allaah [The Most High]; however Fir’awn thought that he is honoured and Musa is dishonoured and disgraced. He also proclaimed that in addition to this, Musa cannot even speak clearly because he is not eloquent! However, Firawn’s insult against Musa that Musa cannot even  express himself is not something blameworthy at all, because he was able to convey the message, even though he used to encounter some difficulty when speaking.

[An Excerpt from ‘Tayseer Al-kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan’ by Imaam As-Sadi (may Allaah have mercy upon him)]