What is the letter in surah al fathiha that the prophet peace be upon him used to recite with blowing out two cheeks and us muslims can say it with blowing out one cheek?

Question
What is the letter in surah al fathiha that the prophet peace be upon him used to recite with blowing out two cheeks and us Muslims can say it with blowing out one cheek?

this question was asked by son's teacher at his islamic school.

jazakallahu khaire

Answer
Assalaam alaikum.  We can give you the answer the school is looking for, but it is not necessarily correct. 
Some modern tajweed books state that the Prophet, peace and blessings of Allah upon him would recite the letter  from both sides of the tongue (no mention of blowing out the cheeks) and that this is difficult for most Muslims, and usually they use one side of the tongue to pronounce this letter. 
When tajweed scholars searched for the source of this statement however, they cannot find anything authentic, so it should not be used when describing this letter.   As pointed out earlier, blowing out the cheeks has nothing to do with the correct pronunciation of the letter .
The correct definition of this articulation point is: This letter is articulated from the one or both sides of the tongue and from the molars and the gum area next to the molars.  The posterior one third of the side of the tongue is used for this letter.  This letter can be articulated from one side (right or left) of the tongue alone, or from both sides of the tongue simultaneously. 

Please see: http://www.abouttajweed.com/articulation-points-areas-of-the-arabic-letters/the-side-of-the-t.html
http://www.abouttajweed.com/articulation-points-areas-of-the-arabic-letters/the-side-of-the-t.html

Wa lakum al-jazaa'.