Why in Surah al Fatihah in the ayah that reads "Ihdinas-siratal mustaqeem" in Hafs do we not read the "noon" with the "alif" that is immediately after it?

Question
As salaamu alaikum.  Why in Surah al Fatihah in the ayah that reads "Ihdinas-siratal mustaqeem"  in Hafs do we not read the "noon" with the "alif" that is immediately after it?  As I understand, we read it as a "noon" with "fathah" that merges into the "sawd" with "shaddah". 

JazakAllah Khair.

Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh
When there are two saakin (meaning non-voweled)  letters meeting between two words, and the last letter of the first word is a medd letter (alif or lengthened wow, or lengthened ya'), the medd letter is dropped when joining the two words together.  There is a forbiddance in the Arabic language of two saakin letters meeting between two words.  In the case in the above question, the alif is dropped in pronunciation, and we read the following noon with a fathah and go directly to the Saad of  which is the first pronounced letter of the next word. 
For more details, please see:
http://www.abouttajweed.com/121003.htm


Wa iyyaakum wa-l-muslimeen
Wa assalaam