The Deen is complete:
"This day have I perfected your religion for you, completed my favour upon you, and have chosen for you Islaam as your religion" (5:4)
This verse embodies a clear statement that the religion has been completed and so has no need of addition and deletion, and our noble Messenger (sallallaahu `alaihi wasallam) said, "there is nothing that Allaah ordered you with except that I have ordered you with it, and there is nothing that Allaah forbade you from except that I have forbidden you from it" (al-Bayhaqee and others)
And he (sallallaahu `alaihi wasallam) said, "there is nothing that will take you closer to paradise but that I have enjoined it upon you, and there is nothing that will take you closer to hell but that I have warned you from it" (Musnad ash-Shaafi'ee and others).
So the Prophet Muhammad (sallallaahu `alaihi wasallam) fulfilled the trust placed upon him by Allaah completely, explaining the religion to the people in it's inward and outward form, making clear the lawful from the prohibited, the encouraged from the discouraged, even to the extent that he taught us the manners of going to the toilet, the manners of having sex with our partners etc.
Allaah says in His Book,
"And We have revealed to you a Book explaining everything" (16:89),
and He says,
"We have neglected nothing in the Book" (6:37).
So in the Qur'aan is explained the principles of the religion, tawheed in all it's various categories, even to the extent that it teaches us the manners of sitting in circles, visiting other peoples houses, the correct clothing etc.
The Importance of Sticking to the Sunnah:
"Indeed you have in the Messenger of Allaah an excellent example for the one who hopes in Allaah and looks to the Last Day." (33:21), He also says, "so if you obey him (i.e. Muhammad), then you will be guided" (25:54)
The Messenger of Allaah said, "I have left behind me two things, if you cling to them you will never go astray. They are the Book of Allaah and my Sunnah" (al-Haakim)
And he (sallallaahu `alaihi wasallam) also said, "and you will live to see great difference, so cling to my sunnah and the sunnah of the rightly guided caliphs after me, even if it be with your teeth" (Bukhaaree)
The Taabi'ee, the Imaam, az-Zuhree commented, "clinging to the sunnah is to be saved, as Imaam Maalik said, 'like the Ark of Noah', he who embarked upon it was saved, and he who did not was destroyed." (ad-Daarimee)
In these narrations is clear evidence that the way of salvation and safety is to closely follow the Sunnah of our Prophet (sallallaahu `alaihi wasallam) , and not to deviate from his path, not accepting the guidance of another individual, nor accepting a corrupted form of the Sunnah. Allaah says,
"And whosoever contends with the Messenger after guidance has been made clear to him, and follows a way other than the way of the believers, We shall leave in the path he has chosen, and land him in Hell, what an evil destination!" (4:115)
The Sunnah shall become corrupted:
We are taught in the revelation that the sunnah shall become corrupted and this corruption would become the norm, so much so that the people who follow the sunnah would become as strangers, and those that call to purifying the sunnah would be slandered and reviled. The Prophet (sallallaahu `alaihi wasallam) said, "Islaam began as something strange, and it shall return as something strange as it began. So Tooba (a tree in paradise) to the strangers." It was asked, "who are the strangers?" He replied, "those that purify and correct what the people have corrupted of my sunnah" (Tabaraanee in al-Kabeer [6/202])
And what is this corruption? It is none other than innovation, introducing into the pure religion of Allaah new ways of worshipping Him that He has not taught, blemishing His religion with our whims and desires, something that Allaah Himslef condemns,
"Or do they have partners that legislate some religion which Allaah has not given permission for?" (42:21)
The Companion, Abdullaah ibn Mas'ud laments, "how will it be when the trials overcome you, in which the young grow old and the old grow senile. And the people take the bid'ah as the sunnah, and when it changes they say: the sunnah has changed. It was said: when will this be O Abu Abdurrahmaan? He replied: when your speakers are many and your scholars are few, and the wealthy ones are plenty and the trustworthy ones are few." (ad-Daarimee)
How true this statement is today, that the innovations have become so widespread in the religion that the people have taken then to be the sunnah. And when the true scholar speaks out against these innovations he is labelled as a 'wahhabi' by the people because to them it seems that he is changing the sunnah. To Allah we complain of the ignorance of our times!
Only Allaah (SWT) has the right to tell us how to worship Him: