I was curious to know why people give citation from hadith? I have no intention to say that any of the hadith is false.
The Quran will surely not be changed but like the other books changed, can't there be a fitna over hadith changing?
Qur'an was preserved by divine promise. And the Qur'an says:
Say, [O Muhammad], "If you should love Allah , then follow me, [so] Allah will love you and forgive you your sins. And Allah is Forgiving and Merciful." Surah Al-e-Imran, verse 31.
This means that our legislation comes first from Allah in Qur'an, and then from hadith of the Prophet, peace be upon him. Allah says:
And if Muhammad had made up about Us some [false] sayings, We would have seized him by the right hand; Then We would have cut from him the aorta. Surah Al-Haqqah, verses 44-46
Based on this, hadith occupies a very, very, very high status in Islam. The preservation was guaranteed through the science of hadith, which is an excruciatingly detailed study of every narrator of every hadith (their life, biography, memory, manners, etc.) as well as text of every narration.
Also, Allah refers to what the Prophet (peace be upon him) says as a form of wahi (revelation) Surah Najm, verses 3-4.
And in another place in the Qur'an, He promises to preserve the dhikr (sunnah, hadith) that has been revealed to the Prophet (peace be upon him). So the preservation of the sunnah is something that must happen, and we know this both by revelation and logic (how can Allah command us to follow something that isn't preserved?).
Also in the Qur'an: "Obey Allah and the Prophet." Surah Al-e-Imran, verse 32.
So there is no issue about hadith changing. The only issue is that some anti-Islamic sources fake and invent hadith; these are called "mawdoo'", or forged hadith, which are rejected.
Hadith also usually detail specifics. For instance, the Qur'an says, "pray in the morning and the evening." But it does not detail the specifics of how to pray; that comes from hadith.
Also, as Ershad says, during the time where Bukhari, Muslim, et. all started collecting and compiling ahadith together, they were extremely cautious about who to trust. An anecdote from life of Imam Bukhari follows:
Bukhari once traveled several months to the place of a hadith narrator to gather hadith from him. To make the long story short, when he reached his place, he saw the narrator pretending to offer a cat some food (while in reality, there was no food in his hands). Imam Bukhari doubted the narrator's honesty and didn't take any narration from him and return back home though he traveled months for this one person's hadiths.
For more details, consult the science of hadith. It's very deep and vast.