Sources from Rambam

After the court of Rav Ashe, who authored and completed the Talmud during his son's lifetime, the Israelites were dispersed all over the land in extreme measure, and have reached the end of the world and the far islands. There were also great wars and the roads were blocked by armies, and the level of Torah learning has diminished, and the Israelites did not gather in their Yeshivot to study by the tens of thousands as it was done before.

Rather some individuals, the remnants named by G-d (see Yoel 3:5), would gather in each town and country and study Torah, and master the works of all the sages, and from them they learned the way of the law.

Each court which was established after the Talmud in any country, and made decrees, regulations, or customs, to its country or other countries, its decision have not spread through the whole people - because of the dispersion and the difficulty of traveling. Also, that court was considered an individual since the great court, of seventy members, has been annulled many years prior to the canonization of the Talmud.

Therefore we cannot force the residents of one country to follow the custom of another country, and we do not tell a court to make a decree similar to the one made by a different court [even] in the same country. Also, if one of the Geonim taught the way of the law in one manner, and a later court understood that it differs from the way of the law as written in the Talmud, we do not follow the earlier, but rather to the one whose argument is more convincing, regardless if he an earlier or later sage.

These rules apply to the laws, decrees, regulations, and customs, introduced after eh canonization of the Talmud. However, all matters mentioned on the Babylonian Talmud are binding to all Israelites. We force all cities and all countries to follow all customs followed by the Talmudic sages, and to uphold their decrees and their regulations.

[That is] because all these matters in the Talmud were agreed upon by all Israel, and the sages who regulated or decreed or made a custom or judged and taught that the law is such and such, are all the sages of israel or their majority, and they are the ones who received the whole transmission, person from person, all the way to Moshe at Sinai.

All the sages who lived after the canonization of the Talmud and analyzed it, and made a name for themselves through their wisdom - are called geonim. And all these Geonim who lived in Israel, Shinar [Babylonia], Spain and France taught the way of the Talmud, revealed its mysteries and explained its matters, since its path is very deep [hard to comprehend]. Also, it is written in Aramaic with a mixture of other languages, since that language was the French Language of Shinar at the time of the completion of the Talmud, but in other places and even in Shinar at the time of the Geonim, one would not know this language unless it was taught to him.

The people of each city would ask many question of their contemporary Gaon, requesting explanation for the difficult matters of the Talmud, and the [Geonim] answered in accordance with their wisdom. The addressees would then gather the responses into books they would use for studying.

The Geonim of each generation also wrote books to explain the Talmud, some explained single units, some explained specific chapters which present problems to his contemporary students, and some explained whole tractates and orders.

They also wrote clear-cut laws, on timely matters of the forbidden and the permissible, the punishable and the exempt, so those who cannot fathom the Talmud will be closer to understanding [the laws]. This is the holy work in which all Israelite Geonim were engaged in, from the canonization of the Talmud until today, 1108 years after the destruction (1178 CE).

And now the calamities are many and frequent, and the harsh times destroyed everything. The wisdom of our sages has been lost, and the intelligence of our scholars has been hidden [or: has collapsed]. Therefore, those explanations, responses, and laws written by the Geonim have become hard to decipher, and there are only few who can fully comprehend their contents. Needless to say, [fewer comprehend] the Talmud itself: the Babylonian, the Jerusalem Talmud, the Sifra, Sifre and the Toseftas - which all require breadth of mind, a wise soul, and lengthy time before one can conclude from them what is the correct way in the matters of the forbidden and the permissible and the rest of the laws of the Torah.

Because of all that I, Moshe son of Ribbi Maimon of Sepharad, have decided to take action. I have relied on the Creator and analyzed all these writings. I found it necessary to write the conclusions which are derived from all these works; in matters of the forbidden and the permissible, the pure and the impure, with the rest of the laws of the Torah. All will be written in lucid language and few words, so that all people will be well versed in the totality of the Oral Law - with no questions and answers, with no "this one says so and that one says so" but rather clear, accessible, correct concepts, following the law distilled from all the books and commentaries created from the time of Ribbi Judah the Holy until today.

In that manner all the laws will be revealed to children and adults alike [or the lesser and the greater], regarding the laws of each Mitzvah, and the laws of all matters established by the sages and the prophets. In summa [the purpose of my book is] that one will not need any other book regarding the Laws of Israel, but rather this book will gather the whole oral Law, with the regulations, customs, and decrees made from the time of Moshe until the canonization of the Talmud, as the Geonim have explained in all their works which they wrote after he canonization of the Talmud. That is why I named my book Mishne Torah [Second to the Torah] - because one reads first the Written Law and then reads this [book], and from it he knows the whole oral Law, and he does not need to read any other book between them.