The promise made by Allah (SWT) in Qur'an 15:9 is obviously fulfilled in the undisputed purity of the Qur'anic text throughout the fourteen centuries since its revelation. However, what is often forgotten by many Muslims is that the divine promise also includes, by necessity, the Sunnah of the Prophet (PBUH), because the Sunnah is the practical example of the implementation of the Qur'anic guidance, the wisdom taught to the Prophet (PBUH) along with the scripture, and neither the Qur'an nor the Sunnah can be understood correctly without the other.

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.

Fiqh-us-Sunnah Online

Suhaib Hassa of London's Al-Quran Society offers an analysis of the reporting, classification and history of the hadith of Islam.