The Hadith is the record of the sayings of Prophet Muhammad (pbuh). The sayings and conduct of Prophet Muhammad (pbuh) constitute the Sunnah.
Allah (SWT) preserved the Sunnah by enabling the companions and those after them to memorize, write down and pass on the statements of the Prophet (PBUH), and the descriptions of his way, as well as to continue the blessings of practicing the Sunnah.
Later, as the purity of the knowledge of the Sunnah became
threatened, Allah (SWT) caused the Muslim Ummah to produce individuals
with exceptional memory skills and analytical expertise, who
travelled tirelessly to collect thousands of narrations and
distinguish the true words of prophetic wisdom from those corrupted
by weak memories, from forgeries by unscrupulous liars, and
from the statements of the large number of Ulama (scholars),
the companions and those who followed their way. All of this
was achieved through precise attention to the words narrated,
and detailed familiarity with the biographies of the thousands
of reporters of hadith.
The Hadith has come to supplement the Holy Quran as a source
of the Islamic religious law. The Hadith is the second pillar
after the Quran upon which every Muslim rests his faith.
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.