Is that which is forbidden (Muharram) what the Shari'a has decisively prohibited, or what the Messenger (peace and blessings be upon him) did not do, and is everything that the Messenger (peace and blessings be upon him) did not do forbidden?

The forbidden (haram) is that which the Lawgiver has definitively prohibited, meaning a definitive command to abstain. It is incorrect to say that everything the Prophet, may Allah bless him and grant him peace, did not do is forbidden, for the Prophet, may Allah bless him and grant him peace, might abstain from something permissible. The default rule for things is permissibility unless there is a valid legal text (daleel shar'i) proving their prohibition, based on the Almighty’s saying: "He it is Who created for you all that is in the earth." It is not permissible for a person to speak about Allah's law without knowledge, based on the Almighty’s saying: "And do not say about what your tongues falsely describe, 'This is lawful and this is unlawful,' to invent a lie against Allah. Indeed, those who invent a lie against Allah will not succeed."