Is saying "the Messenger did not say that" – in a moment of anger, ignorance, or obstinacy – in response to a citation of a prophetic hadith, a major sin that necessitates repentance and the application of the warning of being seated from the Fire?

As long as you have repented of what you did, Allah Almighty accepts your repentance and forgives your sin, for the one who repents of a sin is like one who has no sin. You are not required to do anything more than sincere repentance. We warn you against persisting in blameworthy disputation and argumentation, for disputation is blameworthy even if you are in the right, as the Prophet, peace and blessings be upon him, said: "I guarantee a house in the outskirts of Paradise for whoever abandons disputation even if he is in the right."