What is the ruling on elongating the word "Amen" after Surat Al-Fatiha in prayer for more than six counts?

Saying "Amin" after Al-Fatiha and prolonging it is a Sunnah from the Prophet, peace and blessings be upon him. It is authentically reported that he recited " ولا الضالين (nor those who go astray)," and then said, "Amin," prolonging his voice with it. Exaggerating the prolongation is disliked (makruh) because it takes the recitation out of its proper context, but it does not invalidate the prayer. However, geminating the letter 'Mim' in "Amin" is forbidden (haram) and invalidates the prayer because it changes the meaning to "qasidin" (we are heading/intending).