Why do we not take the Qur’an alone as the Muslim’s constitution? And is there a contradiction between it and the Sunnah in matters such as the torment of the grave and the lashing of the married and unmarried fornicator?

Allah has made it obligatory to adhere to what the Messenger of Allah, peace and blessings be upon him, brought. If the authentic hadiths seemingly contradict the Qur'an, one should refer to the principles of jurisprudence regarding contradiction and preference. The Qur'an does not contain anything that negates the torment of the grave or the punishment for fornication (exile and stoning). Rather, it contains what indicates the occurrence of the torment of the grave, and stoning was affirmed by a verse whose wording was abrogated but its ruling remained.