O Mu' adh! Do you know what is the Right of Allah over His servants and what is the right of the servants over Allah? Mu'adh said, Allah and His Messenger know best. He (peace be upon him) said, Verily, the Right of Allah over His servants is that they should worship Him, associating nothing with Him. And the right of the servants over Allah is not to punish those who associate nothing with Him.
It is reported in Sahih of AlBukhary on the authority of Ibn Mas'ud (may Allah be pleased with him) that the Prophet (peace be upon him) said, Whoever dies while invoking anything other than Allah as a partner (to Allah), will enter Hell.
Also, Muslim narrated in his Sahihon the authority of Jabir (may Allah be pleased with him) that the Prophet (peace be upon him) said, Whoever meets Allah while associating nothing with Him, will enter Paradise; and whoever meets Him while associating anything with Him, will enter Hell.
There are many other Hadiths in this regard. In fact, the issue of belief in the Oneness of Allah (may He be exalted) and avoiding associates with Allah in His Divinity or worship is the most important issue. Allah (may He be exalted) sent His Messenger Muhammad (peace be upon him) to call people to Tawhid (belief in the Oneness of Allah) and avoid Shirk. The Prophet (peace be upon him) spared no effort in the proclamation of the Message which Allah revealed to him. He (peace be upon him) carried it out at best and suffered severe harm for the Sake of Allah. The Prophet and his Sahabah (companions) (may Allah be pleased with them) showed patience in the proclamation of Da'wah (call to Allah). The result was that Allah removed from the Arabian Peninsula every symbol of Shirk. Thus, people entered into the Religion of Allah in multitudes. Moreover, the idols that surrounded the Ka'bah and were inside it were destroyed. The idols of AlLat, Al'Uzza and Manat (names of the prominent idols that were worshipped by Arabs before Islam), and all other idols of the Arab tribes were devastated. Finally, the Word of Allah became superior to anything else and Islam spread in the Arabian Peninsula. Then Muslims undertook the duty of calling people to the Way of Allah and defended the Cause of Allah outside AlJazirah (Arabian Peninsula). Thus, Allah guided through them those predestined to eternal happiness.