1,001 Sayings and Deeds of the Prophet
Any study beside that of the Quran is a distraction, except the Hadith and jurisprudence in the religion. Knowledge is what He narrated to us, and anything other than that is the whispering of the Satan.
Al-Qaeda's credo as stated in the December 2000 edition of the Taliban's English-language magazine The Islamic Emirate.
Within the house of Islam, the penalty for learning too much about the world—so as to call the tenets of the faith into question—is death. While the Koran merely describes the punishment that awaits the apostate in the next world, the hadith is emphatic about the justice that must be meted out in this one: “Whoever changes his religion, kill him.”
Given the fact that [hadiths are] often used as the lens through which to interpret the Koran, many Muslim jurists consider [them] to be even a greater authority on the practice of Islam.
Sam Harris, The End of Faith - Religion, Terror and the Future of Reason, 2004, W. W. Norton & Company.
Islam is not so much a religion as a way of life with thousands of indelible rules which instruct every waking moment of a believer’s existence.
First, there are God's revealed rules, those of the Koran which we have talked about in Pain, Pleasure and Prejudice, and then there are the hadiths (or Hadith, in English academic usage hadith is often both singular and plural), the sayings and example i.e. deeds of the Prophet Muhammad.
If you are the Taliban or Al-Qaeda, the Koran and the hadiths define Islamic law as in "jurisprudence in the religion", and comprise all you need to know about the nature of your existence and how to lead a God-fearing life.
In learning about the hadiths, you will also get acquainted with the real Muhammad, the Prophet as his closest friends and his child-bride Aisha remember him.