It is an obligation upon every Muslim to know the meaning of “And I bear witness that Muhammad is His slave and messenger.” So when we say “And I bear witness that Muhammad is His slave and messenger” it requires, among other things, that we believe that Muhammad (Sallallaahu ‘alaihi wasallam) conveyed the message and that he carried out his task in its complete and perfect form, such that no one that comes after him, however high or elevated he may be, can correct or amend some part of it.
This is indicated in the statement of Allah:
“This day I have completed your Religion for you, and I have perfected My favor upon you, and I am pleased with Islaam as a Religion for you.” [Surat-ul-Maa’idah: 3]
And the statement of the messenger of Allah (sallallahu alayhi wasallam):
“I have not left anything that will bring you closer to Allaah and distance you from the Hellfire, except that I have commanded you with it. And I have not left anything that will bring you closer to the Hellfire and distance you from Allaah except that I have forbade you from it.”
So nothing can be added, taken away, or amended from the religion. This is why Imaam Maalik (may Allaah have mercy on him) said:
“Whosoever introduces into Islaam an innovation, which he deems is good, then he has claimed that Muhammad (Sallallaahu ‘alaihi wa Sallam) has betrayed (the trust of conveying) the Message. Read the saying of Allaah, the Mighty and Majestic: ‘This day I have completed your Religion for you, and I have perfected My favor upon you, and I am pleased with Islaam as a Religion for you.’ [Surat-ul-Maa’idah:3] So whatever was not (part of) the Religion on that day, is not (part of) the Religion on this day. And the last part of this ummah (nation) will not be rectified, except by that which rectified its first part.”
From this we understand that whoever introduces an innovation, and claims that it is something good, has claimed that the messenger of Allah (sallallahu alayhi wasallam) has betrayed the trust of conveying the message (i.e. He didn’t convey the message completely) because they are doing something good which he (sallallahu alayhi wassalam) didn’t, and he was sent to convey the religion.
We must follow the prophet (sallallahu alayhi wasallam) to be truthful in our claim that he is Allahs slave and final messenger, and that he conveyed the religion in its complete and perfect form. So we must worship Allah only upon that which the prophet Muhammad (sallallahu alayhi wasallam) taught, as He was the most fearful of Allah, the most knowledgable of Allah, and the best of worshipers.
For someone to perform an act of worship that the prophet (sallallahu alayhi wasallam) didn’t do and then say that it is good, is claiming that they have more knowledge of what is better than the prophet (sallallahu alayhi wasallam) and that they are better worshipers than the prophet (sallallahu alayhi wassalam).
So when a person understands this, he will realize the importance of worshiping Allah only upon what was reported from the prophet (sallallahu alayhi wassalam), so as to be truthfull in his claim that, ‘Muhammad is the slave and final messenger of Allah.’
[Source: http://abdurrahman.org/innovation/everyinnovationmisguidance.html ]