Is it obligatory to do everything commanded in a truthful dream, and is failing to do so considered a sin?

A dream does not obligate anything, nor can a legal ruling be based upon it, nor can a right be established by it. Indeed, Imam An-Nawawi narrated from Qadi Iyad the consensus of scholars that a dream does not invalidate an established Sunnah, nor does it establish a Sunnah that was not established, nor does it alter what has been determined in Islamic law. An example of this is if a person sees the Prophet, peace be upon him, in a dream informing him that the thirtieth night of Sha'ban is the first day of Ramadan, then fasting based on this dream is not permissible for the dreamer or for anyone else.