What should a preacher do who, in a Friday sermon, cited the Hadith: "The demolition of the Kaaba is less significant to Allah than the killing of a Muslim," then discovered that it is unauthentic? Is he sinful for what he said, and and is he obliged to inform the congregants of his error?

Returning to the truth is obligatory if a Muslim realizes an error, especially if the error pertains to a Prophetic hadith or a legal ruling. Sin is lifted for unintentional errors, as Allah Almighty says: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended," and as the Prophet, peace and blessings be upon him, said: "Indeed, Allah has overlooked for my Ummah their mistakes, their forgetfulness, and what they are forced to do." Furthermore, the sanctity of a believer is greater in the sight of Allah than the sanctity of the Kaaba, as stated in the authentic hadith.