but hadith has mostly been collected much later than the Quran, who was scattered as a lot of parts or memorized by many memorizers. God or Allah himself promised to preserve Quran (15:9), but nobody can say the same about hadith, even if there are efforts to separate the wheat from the chaff. So hadith should always be well examined before accepting it, and this means in first place does a single hadith goesgo ahead with the teachings of the Quran! The earlier generations of Muslims did so, they even never memorized a word or a new verse of the Quran until they understood it and applied it in their real life, and so did the next generations with hadith. But the problem with hadith is that with the time hadith fabrication became a big issue, as an orally transmitted word can to some extent be easily manipulated, and many scholars just collected hadith and compiled it in books or transmitted it without (or with only minor) further verification. That's why among sunni's there's a big dispute between the people of fiqh (Islamic jurisprudence) and the people of hadith! ItFor the hadith scholars it took about 3 centuries to start examining hadith again and Muslims do this to some extent and should do it all along.