Why we do ghunna when we are reading quran?

Assalamoalikum,
 Why we do ghunna when we are reading Qur'an?

Wa alaikum assalam wa rahmatullahi wa barakatuh

As Muslims, we believe the Qur'an was revealed to the Messenger of Allah, peace and blessings of Allah upon him, and the companions of the Prophet learned it from him, just as he recited.  They then passed it down in authentic chains over the generations to us this day.  We have many Muslim brothers and sisters who have an authentic sanad linking them with their correct recitation one by one through the generations to the Prophet Muhammed, peace and blessings of Allah upon him. 

The Prophet peace and blessings of Allah upon him read with the application of ghunnah according to the rules we now have which outline when we should use the nasalization (ghunnah) in our recitation.

The ghunnah read correctly is part of the original recitation of the Qur'an.  It should be applied in our recitation according to the rules of tajweed, and not by our own whims. 

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