Answer: from the overwhelming and undeniable consensus of the Muslims. The Muslims were taught to pray these prayers in this way by the Prophet, and every generation have taught the next to pray like that ever since -- without any protests or disagreement. This is enough, but there are of course many hadiths.
Niklas asked: how to pray only following al quran not sunnah?
Answer: You can't. The Quran, with only a few exception gives general, not specific orders to humankind, such as "Pray!", or "Pay zakaat!". A noted exception here is inheritance law, where there are many details mentioned in the Quran. This is for the obvious wisdom that any differences in understanding these laws would have led to a great deal of disunity.
The details of how to follow these general orders in the Quran was taught by the Prophet Muhammad (may Allah raise his rank). The Quran orders us to obey the Prophet, so following the sunnah is to follow the Quran. The one who refuses to follow the sunnah (what was related to us from the Prophet of his teachings) has denied the Quran. Take a look at the article The Foundations of the Religion to understand better the relation between reason, belief and practices of worship in Islam.
Niklas asked: 2 do you hanafis have some imams in denmark i can call/talk to?
Answer: I don't know of any. However, I'll be in Norway within the next few weeks if you have a chance to meet, or would like to call. Let me know.