Is the narration true regarding the debate that took place between Imam Ahmad and Imam Al-Shafi'i concerning the ruling on one who abandons prayer, and that Ahmad was cut off after Al-Shafi'i's question: "The prayer of a disbeliever is not valid, nor is Islam affirmed by it"?

This debate is not sound in its chain of transmission (sanad) or its text (matn). Al-Albani and Al-Subki pointed to its lack of authenticity and its discontinuity. Furthermore, Al-Shafi'i and Ahmad are too knowledgeable for such a harsh debate to have occurred between them in this manner. It is also well-known in Imam Ahmad's madhhab (definition) that one who disbelieves by abandoning prayer does not become Muslim except by performing it, and not merely by uttering the two testimonies of faith (Shahadatayn). The rule is that one who disbelieves in a specific matter is not sufficed by uttering the two testimonies of faith unless he rectifies the matter through which he disbelieved.