The Prophet Muhammad (saas) explained his people that Allah is the only god and summoned them to believe in Allah. He warned against a grievous torment. Some of the relevant verses are as follows:

Say: “This is my path. With inner sight, I call on you to believe in Allah, I and all who follow me. Glory be to Allah! I am not one of the idolaters!” (Surah Yusuf, 108)

Say: “I call only upon my Lord and do not associate anyone else with Him.” Say: “I have no control over any good or evil that befalls you.” Say: “No one can protect me from Allah and I will never find any refuge apart from Him. My mission is only to make known His Messages.” “As for one who disobeys Allah and His Messenger, he shall abide in the Fire of Hell, remaining in it timelessly, for ever and ever.” (Surat al-Jinn, 20-23)

Say: “I am commanded to serve Allah, and to worship none besides Him. And I am commanded to be the first of the Muslims.” Say: “I fear, were I to disobey my Lord, the punishment of a Terrible Day.” Say: “It is Allah I serve, and Him alone I worship. As for yourselves worship anything you will apart from Him!” Say: “The real losers are those who will lose themselves and their families on the Day of Rising. That shall be the ultimate loss.”(Surat az-Zumar, 11-15)

Surely worthier is He Who originates creation and then regenerates it and provides for you from the Earth and sky. Is there another god besides Allah? Say: “Bring your proof if you are being truthful.” (Surat an-Naml, 64)