Allaah (The Most High) said:

مَا اتَّخَذَ اللَّهُ مِن وَلَدٍ وَمَا كَانَ مَعَهُ مِنْ إِلَٰهٍ ۚ إِذًا لَّذَهَبَ كُلُّ إِلَٰهٍ بِمَا خَلَقَ وَلَعَلَا بَعْضُهُمْ عَلَىٰ بَعْضٍ ۚ سُبْحَانَ اللَّهِ عَمَّا يَصِفُونَ

‘’No son (or offspring or children) did Allah beget, nor is there any ilah (god) along with Him; (if there had been many gods), behold, each god would have taken away what he had created, and some would have tried to overcome others! Glorified be Allah above all that they attribute to Him!’’ [Quran, 23:91]

Imaam Ibnul Qayyim (rahimahullaah) said:

Ponder upon this overwhelming proof based on this short and clear statement; for indeed a true god has to be a Creator and a performer of actions-benefitting the slaves and averting evil from them. Had it been that there was another god alongside [Allaah (The One Free from All Imperfections)], then that god would have had his own creation and actions, but [Allaah (The One Free from All Imperfections)] is not pleased that another god is associated as a partner of His. That is because (if such a god was existent) and able to overcome Allaah, he would have done so. And if (such a god) was unable to overcome Allaah, he would have taken away what he created and depart, just as the kings of this world isolate themselves from one another when they are unable to overcome and gain authority over others. Therefore, there has to be one of the (following) three affairs: Either that each god departs with what he has created and with his authority; or that some of them gain authority over others, or that all of them are placed under the authority of one god, who controls the affairs and they do not control His affairs, and they are restricted by His judgements, but they cannot restrict Him. So Allaah He (alone) is the true God and they are His nurtured and subjugated slaves.

The organization of the affairs of the world- above and below- the connection between them and their establishment upon a well-planned and perfect organization neither clashes nor becomes ruin. This is the most confirmed evidence that the controller of its affairs is one (i.e. Allaah) and that there is not another god besides Him; just as evidence makes it impossible for its creator to be more than one and that there is not another creator for it. And just as it is impossible for the universe to have another Lord and creator, likewise it is not possible for it to have another god that has the right to be worshipped (except Allaah). [Source: Badaa-i’ul At-Tafseer Al-Jaami Limaa Fassarahu Al-Imaam Ibnu Qayyim Al-Jawziyyah: page: 376: Vol: 1…slightly paraphrased and abridged]