As far as i believe ALLAH said “Quran is a divine knowledge for worldly beings (Ashraf ul Makhlooq)” and “He Himself has taken its responsibility” i-e none of its word can be altered or changed. Except Quran every book in this world has tendency to be changed or altered. Most of the changes occurred in Torah, Injeel and Zabur because they were not direct wordings of ALLAH but they are the teachings of “Prophets” written by Ravi (third person / scholars of their time), in a manner they were perceived or understood by them with respect to the situations or circumstances. Whereas in Quran we see ALLAH directly addresses His audience “Tau tum apnay Rab ki kon kon si naimat ko jhutlao gy”. Whereas in Bible (Torah, Zabur, injeel) and Hadith third person is the writer who listens to it by a Prophet or sees Prophet doing it and then write it for coming generations with the interpretations they understood.
As in Surah Al Nisa (65) in Quran it is written that “none of you can become a true believer until you don’t follow the teachings of Prophets”
Now if we Study Surah Al-Nisa it contains instructions for believers from all generations which already existed or has to come.
And it’s written that your belief in ALLAH is incomplete until you don’t believe in all of (124 lac) prophets and angels descended for your guidance.
But as Reference : Anas ibn Malik reported: The Messenger of Allah, peace and blessings be upon him, said, “None of you have faith until I am more beloved to him than his children, his father, and all of the people” Ravi of this Hadith quoted it as a reference for Prophet Muhammad SAW particularly. As we can see this hadith has a solid connection to the teachings of Quran and its is completely true that our Iman is incomplete without following teachings of our Last Prophet Muhammad SAW. But Hadith specifies this narration towards their particular situation and circumstances, as Prophet Muhammad was the only present Prophet at that time. But this Hadith has limited its concept to Prophet Muhammad SAW, just like the teachings of Torah, zabur and injeel are particular to their respective Prophets i-e Prophet Musa, Prophet Daud and Prophet Isa.
So if you want to get a holistic perspective of ALLAH’s teachings (regarding all religions, all prophets, from beginning to the end of this world) then follow Quran (descended on our Last Prophet SAW). But if you want to make decision based on situations like faced by our Last Prophet Muhammad SAW then follow Hadith (written by Sahabas after seeing or listening our last Prophet Muhammad SAW while performing duties of ALLAH).