Why is there no idghaam of the noon in the 'MA NANSAKH" in surat Al Baqara ayat 106?

Question
Assalamu Alaikum

Please explain why there is no idhgham of noon in the kalimna 'MA NANSAKH" in surat Al Baqara ayat 106?

Answer

Wa alaikum assalaam wa rahmatullahi wa barakatuh.
There are specific letters that must follow a saakinah or tanween for an idhgaam to occur.  In the word in question, the first has a vowel, so it is not considered for a rule since the rules are only for saakinah and tanween, not for voweled letters.  The word "saakinah" means non-voweled.
The second in the word in the question is saakinah, so here there is going to be a rule.  The letters that cause idghaam for the saakinah and tanween, meaning they follow the saakinah, are the letters in the phrase: .  The letter following the non-voweled (saakinah) in the word nansakh is not one of these letters, instead it is the letter .  The letter  is one of the letters that cause ikhfa' or hiding of the saakinah and tanween.  Therefore the rule here is ikhfa', and there is no idghaam. 
Please also see:
http://www.abouttajweed.com/the-noon-saakinah-and-tanween/index.php