Hadith are a Primary Source of Islamic Teachings
The Qur'an establishes that the Prophet, peace be upon him, must be referred to when disputations occur. The Prophet peace be upon him is a legislator and does not speak out of whim. The Qur'an cannot command Muslims to follow the Prophet without making a means for them to know and follow him. If his teachings are not preserved, then the Qur'an would be commanding Muslims to follow something that does not exist. From this we understand that part of the preservation of the Qur'an is the preservation of the Sunna itself.