The full significance of this message must never escape us. Islam is unlike all previous revealed religions in one crucial respect. All of them came with expiration dates. Islam has none. The Guidance from Allah had been completed. The religion had been perfected. There would be no new message, no new prophet, no new Shariah, and no new command until the Last Day! The Straight Path has been laid out. Our job is only to follow it, not to try to discover new paths. In Jumuah khutbahs this Ummah has been repeating the hadith: "I warn you of the newly invented matters (in the religion), and every newly invented matter is bid'ah, and every bid'ah is misguidance, and every misguidance is in the Hellfire." (an-Nisaa'ee)
In Islamic terminology, Sunnah and Bid'ah are antonyms. Sunnah literally means path, and it is the path shown to us by the Prophet peace be upon him. This includes the Shariah teachings derived from Qur'an, Hadith, the consensus of the companions, and the ijtehad of the qualified imams. Bid'ah means adding or changing articles of faith or religious practices. It can take many forms. One may change the occasion of a prescribed act, thereby extending it to occasions for which it was not meant. One may add restrictions on a desired act that the Shariah had not imposed. One may change the style or form of such an act. One may start doing something collectively that was to be performed individually. Or one may change the Shariah status of an act from permissible to mandatory. Of course, one may also add a ritual where none existed. These are all forms of bid'ah. They are all forbidden.
Bid'ah is like fake currency that tries to drive out the good currency. By design it has the appearance of a virtuous religious act. But it lies outside the Shariah. So do its sources, which, in a great number of cases can be traced to non-Islamic influence from surrounding communities with which Muslim communities historically came into contact. Hence the telltale signs that set it apart from Sunnah. First, bid'ahs normally vary from region to region--- and over time--- revealing their local, non-Islamic source. This is unlike the genuine religious practices that maintain the same form everywhere. No matter where he comes from, a follower of, say, Hanafi Fiqh, will be offering salat in exactly the same way, right down to the minutest detail - like when to raise the index finger. In contrast, the bid'ah practices surrounding, marriage or death in the Indo-Pak subcontinent vary from those in Arabia or Africa.
Second, the bid'ah practices are largely transmitted through oral tradition. Many of these have a pseudo-legal, ritualistic framework of their own, but one would be hard pressed to find it in the standard legal texts! Rather it lives in the folklore.
This leads us to a simple test for determining whether a commonly observed practice is sunnah or bid'ah. If it is performed as a religious ritual, check it out in a reliable book of fiqh. If it is not there, most probably it is not a sunnah. Example: consider the practice of shaking hands after finishing the salat. Open the chapter on salat in your fiqh book. It lists all the steps, in great detail, involved in offering salat. Does it mention the handshake as well? No. There is our clue that it is a bid'ah, which it is. Similarly look at all the rituals normally performed upon the death of a person. Again the fiqh books describe in great detail how the funeral and burial should be done. But do they also mention that on the third day (or the tenth or the fortieth), a gathering should be arranged where participants should recite the Qur'an for the benefit of the deceased and after which they should be served with dinner? Again the answer is no. Again the reason is that all of these common practices are not part of the Shariah. They are an addition or bid'ah.
Of course, this is not a legal principle. (For obvious reasons it can't be for who is to stop someone from writing a book of fiqh that includes the bid'ahs?) Ultimately one has to turn to scholars to determine whether an act is a bid'ah or sunnah. Yet this test can help an ordinary person raise questions about common practices. One factor that helps the propagation of bid'ahs is the attitude that treats religion as hobby rather than as the serious business of submitting to the command of Allah. Pure submission may be "boring." It demands sacrifice. Bid'ahs are fun. On top of that they "promise" reward in the hereafter. This makes the bid'ah more deadly than ordinary sins. From an act we know to be a sin, we can repent. But how can one repent from a wrong that he considers to be right?
But in reality bid'ahs are a tremendous burden. Islamic teachings are simple and easy. When a person dies, Islam teaches that others should be providing food to the bereaved family. Bid'ah requires the exact opposite. Other bid'ahs are also like that. A burden. And the burden in the Hereafter will be much bigger, for "every bid'ah is in the Fire."