Islam has two primary sources.  First is the Quran which is the direct word of God inspired to the Prophet Muhammad, may the mercy and blessings of God be upon him. The second source is the Prophet’s teachings.  These teachings include his words, actions, and things he approved of.  The Prophet’s teachings are called Sunnah.  The Sunnah is found in texts called hadith.  Hadith are statements of the Prophet which were narrated by his Companions and subsequently narrated to the next generation until these sayings were compiled in hadith collections.

The Prophet Muhammad was sent as the final messenger to mankind.  With his death, the message of Islam was completed.  The preservation of scripture is not limited to the text of the Quran, but its meaning as well.  If the Prophet’s explanation is needed in order to understand the Quran, then it is necessary that his sayings be preserved as well, not only the words of the Quran.

Imagine for instance trying to discover the manner of prayer based only on the command to "establish prayer" with few references to bowing and prostration.  The number of prayers, times per day, and what to recite would remain unknown.  Therefore, the Sunnah is part of the Quran’s preservation.

Importance of Hadith

All Muslims, be they Sunni or Shia, agree that hadith are essential to understanding Islam.  They provide a context to the verses in the Quran.  The Quran is a rather concise book and therefore contains many general statements.  For instance, the Quran commands Muslims to pray, but it does not provide the details of how one is to perform prayer.

The Quran also commands Muslims to perform pilgrimage and give charity, but it does not provide the details.  These details are found in the hadith where Prophet Muhammad explained the details and mechanics of prayer as well as other aspects of Islam.  There are dozens of verses in the Quran that command Muslims to follow the Prophet.  Without knowing his teachings, one cannot possibly fulfill this command.

Collection of Hadith

The Companions of Prophet Muhammad memorized his statements and actions.  In addition to memorization, many Companions wrote these hadith down in their personal collections.  These hadith were passed down to the students of the Companions and subsequently down to their students.  Several Muslim scholars collected these hadith into compilations which have become widespread and are the main sources of hadith until today.

Hadith are a Primary Source of Islamic Teachings

The verses concerning the authority of the Prophet’s teachings are abundant; for brevity we will only list four:

"And whatever the Messenger gives you, accept it, and from whatever he forbids you, abstain from it." (Quran 59:7)

"Say: If you love God then follow me and God will love you and forgive your sins." (Quran 3:31)

"Whoever obeys the Messenger has obeyed God." (Quran 4:80)

"O you who believe, obey God and obey the Messenger and those in authority among you.  If you differ in anything, then refer it to God and His Messenger if you believe in God and the Last Day; that is better and the best interpretation." (Quran 4:59)

The Quran establishes that Prophet Muhammad must be referred to when disputations occur.  This is an example of how the Prophet is a legislator and does not speak out of whim.  The Quran cannot command Muslims to follow the Prophet without making a means for them to know and follow him.  If his teachings are not preserved, then the Quran would be commanding Muslims to follow something that does not exist.  From this we understand that part of the preservation of the Quran is the preservation of the Sunnah itself.

Authentication of Hadith

In order to ensure that hadith were authentic and not fabricated, scholars developed a unique and critical method.  This consisted of two components, first scholars scrutinized the people who were narrating the hadith.  They ensured that everyone in the chain of transmission met each other and was free from any disqualifying characteristics.  These disqualifying characteristics include lying, indulging in major sins, or having a known or obvious motive to fabricate a hadith.

The second criteria they used was to measure and grade the memory of the narrators.  This was done empirically by comparing the narrations of different students to see who might have made a mistake.  For instance, a Companion of the Prophet might have narrated ten sayings of the Prophet to 15 students.  Hadith scholars would then individually ask these students to recite the hadith to them.  If of those 15 students 13 students say that the hadith says XYZ and 2 say it says ZYX then it is likely they made a mistake.  They would then note that this narrator has a poor memory and their narrations are to be rejected or accepted with caution.  Hadith would then be classified as authentic, acceptable, weak, or fabricated.  For more details on this process check out this article.

Examples of Prophet’s Statements Found in Hadith

"Those people who show no mercy will receive no mercy from God."[1]

"Not one of you can (truly) believe if you do not want for your (believing) brother what you want for yourself."[2]

"Do not have ill-will towards one another, do not be envious of one another, do not turn your back on one another; O, servants of God, be brothers (and sisters).  It is not permissible for a Muslim to remain angry with their brother [in religion] for more than three days."[3]

"When a human being dies all their actions and blessings come to an end, except for these three things: continuous charity, the knowledge from which they have benefited others, and a blessed child who prays for them."[4]

"If someone conceals the mistake of another servant, on the Day of Judgement God will cover their mistake."[5]

Conclusion

The statements of Prophet Muhammad are the second source of Islamic knowledge and law.  These statements help understand the Quran and explain its application.  Scholars developed a science which they used to determine the level of each hadith’s authenticity.  Hadith serve as a source of guidance for Muslims in their daily lives.  They use hadith to learn more about the Prophet and try to emulate his actions and character.