i read in a book about the sciences of hadith that the quran says that Allah has promised to preserve it, and since the hadith explain the quran so that means that they are included.

collecting hadith was no minor matter. one scholar travelled so far just to here a particular hadith from someone. but when they came and saw them they saw that the person was deceiving their horse. because of this the scholar quietly left without asking the person anything. if the person showed signs of deception with his horse then how could he be trusted with hadith...?