Is he who knowingly attributed false narrations to the Messenger of Allah an infidel? Thank you.

Lying about the Prophet, peace and blessings be upon him, is forbidden and is considered one of the greatest major sins by the consensus of Muslims. The perpetrator is threatened with Hellfire. Some scholars have even considered it an act of disbelief (kufr), and Imam Ahmad was of the opinion that it renders one a fasiq (transgressor), and his testimony and narration are to be rejected. Furthermore, it is not permissible to narrate a fabricated hadith except by clarifying that it is fabricated.