The word "Sunnah" is also used to refer to religious duties that are optional. Here, we are concerned with Sunnah in the sense of the recorded sayings (Hadiths) of Prophet Muhammad (peace and blessings be upon him). In this sense, Hadith is considered to be second to the Qur'an. It is impossible to understand the Qur'an without reference to the Hadith; and it is impossible to explain a hadith without relating it to the Qur'an.
The Qur'an is the message, while the Hadith is the verbal translation of the message into pragmatic terms, as exemplified by the Prophet. While the Qur'an is the metaphysical basis of the Sunnah, the Sunnah is the practical demonstration of the precepts laid down in the Qur'an.
The duty of the Messenger was not just to communicate the message, rather, he was entrusted with the most important task of explaining and illustrating that message. That is the reason why Allah Himself has commanded the following:
[Say: Obey Allah and obey the Messenger, but if you turn away, he (the Prophet) is only responsible for the duty placed on him (i.e. to convey Allah's Message) and you for that placed on you. If you obey him, you shall be on the right guidance. The Messenger's duty is only to convey (the message) in a clear way.] (An-Nur 24:54)
This verse clearly tells us the overriding importance of Hadith to Muslims. They should be eager to learn and follow the teachings of the Prophet as expressed in Hadith. If we are negligent in this respect, it is we who have to answer before Allah.
Speaking of the importance of Hadith, we need to take into consideration two broad aspects of the subject. We know that Allah Almighty revealed the Qur'an to His chosen Prophet Muhammad (peace and blessings be upon him). So it is through the Prophet we come to hear the word of Allah; and it is the Prophet himself who can properly explain and demonstrate the precepts in the Qur'an. Without the required explanations and illustrations given by the Prophet, the Qur'an may be misunderstood and misinterpreted by people. So the Prophet took care to explain and demonstrate to his companions how the Qur'anic verses must be read and understood. That is to say, the importance of Hadith is linked to the importance of the Qur'an.
For example, the details of how to perform salah (ritual prayer), for instance, were given by the Prophet through his words and action, and not by the Qur'an. This means that we wouldn't know how to pray, fast, pay zakah, or perform Hajj without the examples given by the Prophet as recorded in the Hadith. Indeed, all necessary details are given in the Hadith, not in the Qur'an.
The revelation of each of the verses of the Qur'an took place at some critical junctures in the life of the Prophet. Of course, there are verses of universal application and significance, irrespective of the context in which those verses were revealed. But there are other verses that can be understood or interpreted only in the light of the actual context in the life of the Prophet, which called for that revelation. There are many examples. For instance, the following verse in the Surah Aali `Imran:
[If any one disputes in this matter with thee, now after (full) knowledge hath come to thee, Say: Come! Let us gather together, our sons and your sons, our women and your women, ourselves and yourselves: then let us earnestly pray. And invoke the curse of Allah on those who lie.] (Aali `Imran 3:61)
This verse talks about mubahala (invoking the curse of Allah on those who take a dishonest stand); and was revealed when the Prophet was conferring with the Christian delegation from Najran in 631 CE This example clearly shows how we need to refer to the life and example of the Prophet to understand the context, as well as the meaning of verses, such as the above mentioned one in the Qur'an.
The foregoing shows how Hadith, in practical terms, explains, clarifies, and paraphrases the Qur'an. If we reject the Hadith, we may misread the Qur'an; so Hadith is central to a proper understanding of the Qur'an.
In the Qur'an, Allah Almighty commands us not only to obey the Messenger, but also to abide by his decisions as follows:
[But no, by the Lord, they can have no (real) Faith, until they make you (the Prophet] judge in all disputes between them, and find in their souls no resistance against your decisions, but accept them with the fullest conviction."] (An-Nisaa' 4:65)
And surely we find such decisions only in the Hadith; the duty of Muslims is to accept the Prophet's decisions whole-heartedly. The Qur'an also orders the faithful to emulate the role model of the Messenger and reckons it to be the only way to gain the pleasure of Allah.
It is therefore obligatory that we look up to the Prophet's morals and exemplary character and carry them out in our lives. We can never do so without studying Hadith. It is most illuminating in this respect to learn that when `A'ishah (may Allah be pleased with her) was asked to describe the character of the Prophet (peace and blessings be upon him), her definitive answer was, "His character was that of the Qur'an." In other words, Prophet Muhammad (peace and blessings be upon him) personified the best ideals and values of the Qur'an. How could we then neglect the Hadith, which alone can lead us to the precise ways in which the Prophet exemplified the Qur'anic ideals?