How can we prove that saying ameen out loud after the Faatihah was a regular practice of the Prophet, sallallaahu ʻalayhi wa sallam, and not something that he did only at the initial stages of teaching how to perform the prayer (as claimed by the Hanafis)? How can we prove that this practice of saying "ameen" loudly was not abrogated?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

According to the most likely correct view, it is legislated for the imaam and the one being led in the prayer to say Aameen out loud in the audible prayers. This is the scholarly opinion of the Hanbalis and the early view of Ash-Shaafiʻi. It is not true that the Prophet, sallallaahu ʻalayhi wa sallam, used to do so in the initial stages of teaching prayer as some claim. The Companions have narrated that the Prophet, sallallaahu ʻalayhi wa sallam, used to say Aameen out loud in the prayers, and some of them, such as Abu Hurayrah and Waa'il ibn Hujr  converted to Islam late in the lifetime of the Prophet, sallallaahu ʻalayhi wa sallam. This proves that the Prophet, sallallaahu ʻalayhi wa sallam, used to do so until the last days of his noble life. The claims of abrogation are refuted because abrogation is not suggested except in case their dates are known and there is no room for reconciliation between the two seemingly contradictory texts. This is not the case in the matter in reference.

There are many pieces of evidence indicating that it is legislated to say Aameen out loud in the audible prayer. For instance, Abu Hurayrah narrated that the Prophet, sallallaahu ʻalayhi wa sallam, said, “Say Aameen when the imaam says Aameen, for if anyone's utterance of Aameen is simultaneous with that of the angels, he will be forgiven his sins.” [Al-Bukhari and Muslim] Moreover, Waa'il ibn Hujr said, “When the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, recited the verse: {...not of those who have evoked (Your) anger or of those who are astray.} [Quran 1:7], he would say Aameen and raise his voice with it.” [Abu Daawoud and At-Tirmithi] In At-Tirmithi's narration, he said, “... and extend his voice with it.” [At-Tirmithi graded this narration hasan (good)]

The claim of abrogation is also refuted by the practice of the righteous predecessors. ʻAtaa’ said, “Ibn Az-Zubayr would say Aameen, and those who were praying behind him would say Aameen after him so loudly that the masjid would shake with it.”

This clearly indicates that the soundness of the scholarly view suggesting that it is legislated for the imaam and the one being led in the prayer to say Aameen out loud in the audible prayers and that it was not abrogated as claimed. In any case, this is a controversial issue that is open to Ijtihaad (scholarly reasoning).

Allaah knows best.