The authority of Hadith-ul-Ahad

Q: What about a person who denies the punishment in the grave, because the reports in this regard are Hadith-ul-Ahad (Hadiths which at some point in the chain have only a single narrator) which -they hold- can never be taken as basis for beliefs? They do not judge reports in terms of authenticity, acceptability, and weakness, but in terms of the numerous chains of transmission; whether it is a singular report or recurrently reported at each generation of narrators. Upon finding out that a certain report is singular, they reject it. How does one respond to this?

A: All Praise is due to Allah Alone. Peace and blessings be upon His Messenger, his family, and Companions. To commence, if a singular report is authentically reported from Allah's Messenger (peace be upon him), it is a credited proof according to the consensus of Ahl-ul-Sunnah wal-Jama`ah (those adhering to the Sunnah and the Muslim community), whether on issues of worship or creed. Whoever denies the authority of Hadith-ul-Ahad after establishing the evidence against him is a disbeliever. For further explanation in this regard, refer to the book entitled Al-Sawa`iq, by Ibn Al-Qayyim or its abridged version by Al-Mawsily. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.