The word Sunnah and hadīth have the same meaning. The meaning of the word Sunnah is what has been reported from the Messenger of Allah  of his saying, action and consent. Whatever has been reported about the Sahabah  is also considered part of the Sunnah because they used to live with the Prophet , listen to his  saying, witness his  actions and narrate what they saw and heard. The hadīth is considered a Sharī’ah text because Allah  said:

And He  said:

Many ayat of the Qur’ān have come as mujmal (ambivalent) for which the hadīth has provided the details. For example, the ayat related to prayer came as mujmal (ambivalent), but it is the actions of the Prophet  that clarifies the time and manner of prayer. In this way, many of the ahkām in the Qur’ān were revealed as mujmal (ambivalent) and the Messenger  used to explain them. He  said:

The Sahabah  were the ones who heard the sayings of the Prophet  and they saw his  actions and condition. When they came across a problem in understanding an ayah or they disagreed about its tafsīr or a ruling from it, they would refer to the prophetic ahadīth for clarification. In the beginning, the Muslims used to rely on memory and accurate transmission without looking at what they have written, by memorising this knowledge like their memorisation of the Book of Allah . With time Islam spread and the lands grew, and the Sahabah  spread across the regions and most of them died and the accuracy in transmission diminished, it therefore became necessary to document the hadīth and preserve them in writing.

The era of compiling the hadīth goes back to the period of the Sahabah. There were a number of persons amongst them who used to write and narrate from what they had written. It has been narrated about Abu Hurairah  that he said:

However, those Sahabah who did write down the ahadīth were few in number. Most of the Sahabah used to memorise thee ahadith by heart since they were forbidden from writing the hadīth in the beginning of Islam. Muslim reported in his Sahih on the authority of Abu Sa'eed al-Khudri  that he said that the Messenger of Allah  said:

This is the reason why the Sahabah  desisted from writing down ahadīth, and they were content just to rely on memorisation and paying attention. The Sahabah paid careful attention to learning the hadīth. It has been established that many Sahabah  refrained from accepting numerous reports. Ibn Shihab narrated from Qabisa that her grandmother came to Abu Bakr , asking about her rights in inheritance. He said: I did not find anything mentioned in the Qur’ān for you and I do not know that the Messenger of Allah  mentioned anything for you, then he asked the people. al-Mughira stood up and said:

He  said: Do you have anyone who can corroborate this? So Muhammad ibn Maslama  bore witness to the same thing so Abu Bakr  implemented this ruling for her.

Al-Jariri narrated from Abu Nadra who narrated from Abu Sa'eed that Abu Musa greeted Umar with the Salam three times from behind the door but he was not given permission to enter. So he returned. Umar  sent someone for him and asked him why did you return back? He  said: I heard the Messenger of Allah  say:

‘Umar replied: “You must bring me an evidence about this matter otherwise I will punish you.” Abu Musa came to us while we were sitting down and his face was sweaty. We said: “What is the matter with you?” So he informed us and asked: “Did anyone of you hear this Hadith?” We replied: “Yes, all of us have heard this (Hadith).” So we sent a man from amongst us till he came to Umar and told him. Ali  said: “If I heard a Hadith from the Prophet of Allah  which Allah  benefited me with it, and if anyone talked to me about it, I would ask for an oath from him, and if he gave it to me then I would trust him.” From this we see the care of the Sahabah  in the narration of hadīth and the extreme care they excercised in accepting reports. It has even been narrated that Umar  did not give much attention to the narration of Fatimah ibn Qays (which states) that there is no maintenance (nafaqa) or lodging for the woman who has been irrevocably divorced with three pronouncements. He  said: We shall not abandon the Book of our Lord or the Sunnah of our Prophet  for the speech of a woman for we do not know if she has memorised it or forgotten it. This does not mean (that Umar left her hadīth because) she is a woman, rather what it means is that we will not leave the Book and Sunnah for the speech of someone for whom it is not known whether she has memorised it or forgotten it. The illah (reason) is whether she memorised it or not, and not because she was a woman.

When the fitna (civil war) ensued after the murder of Uthman  and Muslims started to disagree among themselves and different groups were formed as a result. The attention of every group was devoted to deducing evidences and reporting ahadīth which supported their claims. Some of them when they needed a hadīth to support a saying or establish proof for something, they would themselves fabricate a hadīth. There was a proliferation of such fabrications during this period of disorder. After the fitna (civil war) had abated the Muslims embarked upon checking the ahadīth, they found that those fabrications had become widespread. So they worked hard to separate the fabrications from the sound (Sahih) ahadīth.

And when the age of the Sahabah  had come to an end and the Tabi'un came after them they followed on the same method and they followed the noble Sahabah  in their attention to the hadīth and its spread through the medium of narration until the reign of the just Khalifah 'Umar ibn 'Abd al-'Aziz in the beginning of the second Hijri century, He ordered the hadīth to be written down. Bukhari said in his Sahih in the kitab al-'ilm (The Book of Knowledge) that 'Umar ibn 'Abd al-'Aziz wrote to Abu Bakr ibn Hazm: 'Look for what you can find of the ahadīth of the Messenger of Allah  and write them down. I fear for the loss of the lessons of knowledge and the dwindling numbers of the scholars. Do not accept anything other than the hadīth of the Prophet  so that you may dissiminate knowledge and sit down to teach those who do not have knowldge until they have knowldge. Verily, knowledge does not perish unless it is kept secret'.