Importance of sincerity and following the teachings of Allah's Messenger
Evidence from the Qur'an on the importance of sincerity and following the teachings of Allah's Messenger (peace and blessings be upon him) which are considered the two conditions for accepting worship:
Allah commanded His believing servants not to associate partners in worshiping Him. He Almighty says: "and invoke Him, sincere to Him in religion." [Al-'Araf: 29]
Allah Almighty is the only Legislator and none is entitled to legislate but Him. Whoever worships Allah through anything which He did not legislate, he thus associates partners with Allah the Exalted. Allah Almighty says: "He has ordained for you of religion what He enjoined upon Noah and that which We have revealed to you, [O Muhammad]." [Ash-Shura: 13]
"And, [moreover], this is My path, which is straight, so follow it; and do not follow [other] ways, for you will be separated from His way." [Al-'An'am: 153]
Allahrebukes whoever brings forth a legislation from his own as per His words: "Or have they other deities who have ordained for them a religion to which Allah has not consented?" [Ash-Shura: 21]
Allah has perfected the religion for us and He chose Islam for us as per His words: "This day those who disbelieve have despaired of [defeating] your religion; so fear them not, but fear Me. This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion." [Al-Ma'ida: 3] Therefore, bringing innovation into the religion is deemed a transgression against Allah and His Messenger and attributing imperfection to His religion.
If it were permittedfor every person to worship Allah according to the way he wishes, life will be unbearable and dispute will prevail due to difference of views which in turn leads to dissention. On the contrary, abandoning innovations and following the way of the Messenger is a great means for achieving agreement and unity.
If it were permitted to people to worship Allah according to their wishes, this will result in doing without Messengers which cannot be accepted by people of discern.
 Excerpted from a pamphlet on Tawhid authored by Sheikh Dr. Abdullah al-Jasir.