Sources of Legislation

We in Islam have three main sources of legislation:
1.The Quran:
Which we believe that it is God's final word to all humanity.
1. Blessed is HE who has sent down Al-Furqan to HIS servant, that he may be a Warner to all the worlds - (Holy Quran 25:1)
God sent the Quran to His servant and Prophet Muhammad (Peace be upon him) through His angel Gabriel distributed within 23 years. It has been tought to the Prophet's disciple, where some of them memorized it, others kept on writing it. Then it was collected in one book immediately after the Prophet's death. And through all ages, there have been many Muslims who memorized the Quran, and those who memorized the Quran had tests to make sure that they thorougly memorized it. And it has been transmitted through generations that way, simultaneously with writing.
2. The Sunna (Hadiths):
Which is the documentation of the Prophet's life, quotes and deeds, what he did, what he ordered us to do, and forbidden us not to do, and every Muslim must follow it since God told us in the Quran:
31. Say `If you love ALLAH, follow me; then will ALLAH love you and forgive you your sins.' And ALLAH is Most Forgiving and Merciful. 32. Say, `Obey ALLAH and HIS Messenger;' but if they turn away, then remember that ALLAH loves not the disbelievers. (Holy Quran 3:31-32)
It had been transmitted through documenting narrations from the Prophet's disciples to generations after them, and there were severe conditions for a hadith to be accepted, the documentation began nearly after 100 years from the Prophet's death, but within this time, anyone who tells any hadith must be known that he is righteous, and that's he doesn't lie at all, and that he memorizes well and doesn't forget quickly, and he must tell who told him that hadith to know if he is really accepted to narrate or not, and there must be evidence that these two narrators really met each other, and there has been documentations about all narrators to know if they are really accepted or not, at the end of every hadith, we have a chain of narrators, for that chain to be accepted, it must be continuous (i.e. each narrator saw the one before him and heard from him), and the narrators must have the conditions mentioned before.
The main books of hadith, are Sahih Bukhari, Sahih Muslim which have no false hadith, then there are Al Tirmithee, Ibn Majah, Al Nasa'i, Abu Dawood, Ahmad, Al Bayhaqi, and other books, these books may have weak or false hadith, scientists examine hadiths, and see if it is really right or good or weak or false, actually the science of Hadith is a huge science, and many books have been written about it.
3. Agreement:
Which means that all Muslim accepted scientists agreed on something, then we consider it a source of legislation, because God said in the Quran:
115. And whoso opposes the Messenger after guidance has become manifest to him, and follows a way other than that of the believers, WE shall let him pursue the way he is pursuing and shall cast him into Hell, and an evil destination it is. (Holy Quran 4:115)
And Muhammad (Peace be upon him) said: "My Ummah (nation) won't agree on mistaking" http://christismuslim.googlepages.com/sourcesoflegislation