Allah (Glory be to Him) protected the Prophetic Sunnah by the efforts of the scholars or Hadith who distinguished its authentic from the not athentic and showed the accepted from the rejected. The Prophetic Sunnah is the explanation to the Glorious Qur'an, and part of protecting the Qur'an is protecting the Sunnah which shows its general rules and details its rulings, and explains its ambiguous meanings.
The Prophet (peace be upon him) used to teach his Companions the Qur'an and the Sunnah as Allah (may He be Exalted) says: "He it is Who sent among the unlettered ones a Messenger (Muhammad peace be upon him) from among themselves, reciting to them His Verses, purifying them (from the filth of disbelief and polytheism), and teaching them the Book (this Quran, Islamic laws and Islamic jurisprudence) and Al-Hikmah (As-Sunnah: legal ways, orders, acts of worship, etc. of Prophet Muhammad peace be upon him). And verily, they had been before in manifest error." [Surat Al Jumu`ah: 2].
The Companions of the Messenger (peace be upon him) paid much attention to the Sunnah as they cared for the Qur'an, but they did not write down the Sunnah in books so as it will not confuse between the words of the Messenger of Allah and the words of Allah. They only memorized Hadiths in their hearts and conveyed them to those who came thereafter as the Prophet (peace be upon him) commanded them to maintain and convey them.
Zayd ibn Thabit (may Allah be pleased with him) narrated: I heard the Messenger of Allah (peace be upon him) saying: "May Allah brighten a man who hears a tradition from us, gets it by heart and passes it on to others. May a bearer of knowledge conveys it to one who is more versed than he is; and may a bearer of knowledge is not versed in it." [Reported by At-Tirmidhy and Abu Dawud, and Al Albany graded it as authentic].
Ahmad and Abu Dawud reported by an authentic chain of narration from at Sulayman ibn Mahran Al A`mash from `Abdullah ibn `Abdullah from Sa`id ibn Jubayr on the authority of Ibn `Abbas (may Allah be pleased with him) who narrated that the Messenger of Allah (peace be upon him) said: "You hear (from me), and others will hear from you; and people will hear from them who heard from you." So, the Prophet (peace be upon him) commanded the Companions to hear then convey it to the Followers then to their successors, and that happened by the will of Allah where the Followers conveyed the Qur'an and the Prophetic Sunnah from the Companions and were sure that they won't get confused with each other, so they started to write down the Sunnah with the chains of transmissions which they heard from their sheikhs. Then their successor exerted efforts in the matter of Hadith and reported Hadiths with different chains of transmission not just one way of narration but from several ways to realize the mistakes which some narrators committed. They wrote great books about the names of narrators to explain his status, faith, and memorization. They knew the status of a narrator by comparing his narrations to the narrations of his counter parts who shared the same narrations from their sheikhs, thus they knew who memorized well from his sheikh and those who added or decreased something to the narration. They also could distinguish any error narrators did in narrations by this scale: Comparing his narration with the narration of another. By this way, they could distinguish odd Hadiths which were reported by a single narrator and judged each Hadith according to acceptance and rejection scale. They also explained authentic Hadiths upon which a person must depend fabricated Hadiths that were reported by liars, and weak Hadiths narrated by untrustworthy narrators who did not memorize Hadiths well, therefore they made mistakes in their narrations because of their bad memory or they did not depend only on their own narrations but add to themselves narrations they did not hear because of their foolishness or lewdness or others as shown in the books of Hadiths.
Of the wisdom of Allah (may He be Exalted) that when he preserved the Qur'an, he preserved the Sunnah which explains it. Allah (may He be Exalted) commanded Muslims with obeying Him as well as His Messenger as He says in the Qur'an: "Obey Allâh and obey the Messenger (Muhammad peace be upon him)." [Surat An-Nisa': 59].
He also says: "He who obeys the Messenger (Muhammad peace be upon him), has indeed obeyed Allâh." [Surat An-Nisa': 80].
He also says: "And whatsoever the Messenger (Muhammad peace be upon him) gives you, take it; and whatsoever he forbids you, abstain (from it)." [Surat Al Hashr: 7].
He also says: "Indeed, in the Messenger of Allâh (Muhammad peace be upon him) you have a good example to follow." [Surat Al Ahzab: 21].
For example, in the Qur'an, Allah commanded us with establishing Salah without explaining to us how to perform it or what are its rulings, but the the Messenger (peace be upon him) explained that in his Sunnah: Showed the number of each Salah, what should be recited while standing, what should be said during bowing down and prostration, and explained its description, rulings, and types. Likewise he did with fasting, Zakat, Hajj, and other rulings, so a Muslim cannot waive the Sunnah. Therefore, Allah preserved the Sunnah for Muslims by the efforts of Hadith scholars as He preserved the Glorious Qur'an for them by the efforts of reciters and memorizers. Praise be to Allah, the Lord of mankind.
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