Doctrine is obtained in Islam from various sources, some more authoritative and direct than others. The first and foremost of these is the miracle and theophany that is the Koran, which is the verbatim Word of God. We have touched on this above.
After the Koran we have the recorded statements of the Prophet, the Hadith, called in the plural ahadith. Shia and Sunni collections of accepted Hadith differ in what they accept as authentic.
Example hadith are:
“No man is a true believer unless he desires for his brother that which he desires for himself.”
“Those people who show no mercy will receive no mercy from Allah.”
“You cannot enter heaven until you believe, and you will not truly believe until you love one another.”
“There are heavenly rewards for every act of kindness to a live animal.”
“The best jihad is the conquest of the self.”
“Heaven lies at the feet of mothers.”
Next there is the comprehensive Sunnah, which means ‘path’ or ‘method’ and which is composed of all the social customs and religious norms established by the Prophet and preserved through tradition. The Hadith is technically a part of the Sunnah, but is more distinct by the form taken, which is that of a direct narration or statement.
Lastly, there is the Sharia, which pertains to law specifically, and is the essence of the rules by which Islamic society is governed. The Koran is the basis of the Sharia, with the Sunna and Hadith being complementary sources that follow the Koran itself.
All of these combine to inform legal policy. As an example, consider the following hadith, which recommends erring on the side of innocence:
“Avoid condemning the Muslim to Hudud whenever you can, and when you can find a way out for the Muslim then release him for it. If the Imam errs it is better that he errs in favor of innocence (pardon) than in favor of guilt (punishment).”
One can see that this also follows the general Islamic saying, “Verily, My Mercy prevaileth over My Wrath.”