Orang Islam sepakat bahawa Sunnah Nabi Muhammad ﷺ ialah sumber kedua daripada dua sumber asas wahyu dalam Islam, selepas Al-Qur'an yang Mulia. Sunnah yang sahih terkandung dalam himpunan besar literatur Hadith.1.

The Muslims are agreed that the Sunnah of the Prophet Muhammad (may Allah bless him and grant him peace) is the second of the two revealed fundamental sources of Islam, after the Glorious Qur'an. The authentic Sunnah is contained within the vast body of Hadith literature.1.

Hadis (jamak: ahadis) terdiri daripada dua bahagian: matan (teks) dan isnad (rangkaian perawi). Sesuatu teks mungkin kelihatan logik dan munasabah, tetapi ia perlu mempunyai isnad yang sahih dengan perawi yang boleh dipercayai barulah ia boleh diterima; 'Abdullah b. al-Mubarak (w. 181 H), salah seorang guru yang terkemuka bagi Imam al-Bukhari, berkata, "Isnad itu sebahagian daripada agama: sekiranya tidak kerana isnad, sesiapa yang mahu pasti akan berkata apa sahaja yang dia suka."2.

A hadith (pl. ahadith) is composed of two parts: the matn (text) and the isnad (chain of reporters). A text may seem to be logical and reasonable but it needs an authentic isnad with reliable reporters to be acceptable; 'Abdullah b. al-Mubarak (d. 181 AH), one of the illustrious teachers of Imam al-Bukhari, said, "The isnad is part of the religion: had it not been for the isnad, whoever wished to would have said whatever he liked."2.

Semasa hayat Nabi (semoga Allah memberkatinya dan mengurniakannya kesejahteraan) dan selepas kewafatannya, para Sahabat (Sahabah) biasa merujuk kepadanya secara langsung apabila memetik sabdanya. Para Tabi'in (Tabi'un) mengikuti jejak tersebut; sebahagian daripada mereka memetik Nabi ﷺ melalui para Sahabat, manakala yang lain meninggalkan perantara itu - hadith sebegini kemudiannya dikenali sebagai mursal. Didapati bahawa mata rantai yang hilang antara Tabi'i dan Nabi ﷺ mungkin seorang sahaja, iaitu seorang Sahabat, atau dua orang, dengan orang tambahan itu ialah seorang Tabi'i yang lebih tua yang mendengar hadith tersebut daripada Sahabat. Ini merupakan contoh bagaimana keperluan untuk mengesahkan setiap isnad timbul; Imam Malik (w. 179) berkata, "Orang pertama yang menggunakan isnad ialah Ibn Shihab al-Zuhri" (w. 124).3.

During the lifetime of the Prophet (may Allah bless him and grant him peace) and after his death, his Companions (Sahabah) used to refer to him directly, when quoting his sayings. The Successors (Tabi'un) followed suit; some of them used to quote the Prophet (may Allah bless him and grant him peace) through the Companions while others would omit the intermediate authority - such a hadith was later known as mursal. It was found that the missing link between the Successor and the Prophet (may Allah bless him and grant him peace) might be one person, i.e. a Companion, or two people, the extra person being an older Successor who heard the hadith from the Companion. This is an example of how the need for the verification of each isnad arose; Imam Malik (d. 179) said, "The first one to utilise the isnad was Ibn Shihab al- Zuhri" (d. 124).3.

Alasan lain yang lebih penting ialah pemalsuan hadith secara sengaja oleh pelbagai golongan yang muncul di kalangan umat Islam, bagi menyokong pandangan mereka (lihat kemudian, di bawah perbincangan mengenai hadith maudu'). Ibn Sirin (w. 110H), seorang Tabi'in, berkata, "Mereka tidak bertanya tentang isnad. Tetapi apabila fitnah (kekacauan, kegawatan, terutamanya perang saudara) berlaku, mereka berkata: Sebutkan kepada kami nama-nama perawi kamu. Maka riwayat Ahl al-Sunnah (Penganut Sunnah) akan diterima, manakala riwayat Ahl al-Bid'ah (Penganut Bid'ah) tidak akan diterima."4.

The other more important reason was the deliberate fabrication of ahadith by various sects which appeared amongst the Muslims, in order to support their views (see later, under discussion of maudu' ahadith). Ibn Sirin (d. 110), a Successor, said, "They would not ask about the isnad. But when the fitnah (trouble, turmoil, esp. civil war) happened, they said: Name to us your men. So the narrations of the Ahl al-Sunnah (Adherents to the Sunnah) would be accepted, while those of the Ahl al-Bid'ah (Adherents to Innovation) would not be accepted."4.