What is the evidence that the prayer of one who intentionally omits audible recitation in an audible prayer is not invalidated, and that audible recitation is a Sunnah, not an obligation, taking into account the command of the Prophet (peace and blessings of Allah be upon him): "Pray as you have seen me pray," and the fact that there is no established instance of him ever omitting audible recitation in an audible prayer?

The hadith "Pray as you have seen me praying" does not indicate the obligation of everything the Prophet, peace and blessings be upon him, did in prayer. Rather, it is understood as obligatory for actions for which another evidence indicates their obligation, and as supererogatory for actions for which an evidence indicates their sunnah status. Most of the Prophet's actions in prayer, peace and blessings be upon him, are sunnah (supererogatory), not obligatory. Reciting aloud in congregational prayers is not obligatory but supererogatory, as there is no evidence to make it obligatory. Therefore, if a worshipper recites Al-Fatiha silently in a congregational prayer, their prayer is valid. The Prophet's consistent practice of an action, peace and blessings be upon him, is not sufficient to establish its obligation, unless other evidence is combined with it that indicates obligation, as in the case of the middle tashahhud, which the Hanbalis deemed obligatory due to a command regarding it in addition to the Prophet's consistent practice of it, peace and blessings be upon him.