What is the ruling on one who narrates weak (da'if) or fabricated (mawdu') hadiths, whether he knows of their weakness or fabrication or not?

It is not permissible to narrate a fabricated (mawdu') hadith unless its fabrication is declared and a warning is issued against acting upon it, due to the Prophet's (peace and blessings be upon him) saying: "Whoever narrates a hadith from me which he deems to be a lie is himself one of the liars," and "Whoever attributes to me what I have not said, let him take his seat in Hellfire."