I am not a scholar, neither am I associative in any Islamic association, so I do not hold my verdict flawless. As I said earlier, many Muslims who had memorised the Quran died in a war, so a complete written and compiled version was to be formed.
After the death of the Prophet Muhammad(P.B.U.H) many people claimed some Hadiths of which several were self created and accociated with the Prophet(P.B.U.H). The first Caliph himself destroyed many such false Hadiths. The critaria that the Muslims follow is as follows.
1: Who is presenting the Hadith? Was he close to the Prophet(P.B.U.H)? How is the person morally, is he a lier, e.t.c.
2: The Hadith presented, are there any other close witnesses to the Hadith?
3: Is there is a contradiction of any sort with the Quran or any other Hadith? It can never by true if it is in contradiction with the Quran.
4: If there is any contradiction with any other Hadith, the presenter who was closer and had more witnesses was considered true. For example, a Hadith put forward by the first Caliph Abu Bakar(R.A), who was among the closest people to the Prophet Muhammad(P.B.U.H), and had several witnesses to it would be more acceptable then one produced by a Muslim who only met the Prophet Muhammad(P.B.U.H) a few times and had on witnesses.
This task was carried out to prevent contradictions and to prevent people from misleading. The similar kind of method was used while compiling the Quran. The written copy was taken from Zaid bin Haris(R.A), a servent of Prophet Muhammad(P.B.U.H) whom the Prophet(P.B.U.H) loved and treated as his(P.B.U.H) son, and was accepted by all the Muslims as the most authentic and true copy. When the other sources or verses were presented, there was a contratiction, the source was not authentic, so the false copy was taken and destroyed. A sample of a part of this non authentic copy is present in a Turkish museum.