What is the evidence that made the majority view the repetition of the Adhan as a Sunnah, even though the original ruling is obligation based on the Prophet's saying: "Then say as he says"?

The scholars differed on the issue of the obligation to repeat after the Mu'adhdhin (caller to prayer). The Hanafis, the Dhahiris, and Ibn Wahb from the Malikis held it to be obligatory, while the majority of scholars considered it recommended. The majority based their argument on the Hadith of Anas in Sahih Muslim that the Prophet, peace and blessings be upon him, when he heard the Adhan (call to prayer), said: "Upon the Fitra" (primordial nature), and "I have emerged from the Fire," and it is not mentioned that he said the same words as the Mu'adhdhin. Ibn Hajar countered this deduction by stating that the Hadith does not explicitly deny that he said the same words; it is possible that he said them but the narrator did not transmit that, or that this occurred before the command to repeat after the Mu'adhdhin was issued.