What is the ruling on uttering speech or supplication during prayer that was not legislated for that specific position?

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This involves saying an utterance or supplication for which no legal basis exists in that part of the prayer, such as saying 'Amin Rabb al-'Alamin' or beginning the *takbir* with 'Allahu Akbar Kabiran' and similar phrases. Prostration of forgetfulness is not legislated for this action because the Prophet (peace be upon him) heard a man say in prayer, 'All praise is due to Allah, praise worthy of Him, good and blessed, as our Lord loves and is pleased with,' and he did not command him to prostrate.