I have heard that it is permitted to recite the sad (as in ihdinaa aS-Siraat) in surah alfatihah with a sin

Question
Assalamu alaykum

I have heard that it is permitted to recite the sad (as in ihdinaa aS-Siraat) in surah alfatihah with a sin (like in biSmi, from the basmalah). I heard that the Prophet (pbuh) permitted this for the person who is unable to say the sad properly. If this is permitted could you please substantiate.

shukran

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh
In the 10 different qira'aat, there are a few riwaayaat in which the word is read and  with a instead of a .  This is not because some could not read the correctly, but it is a riwaayah and associated with specific qira'aat, all with authentic chains back to the Prophet.  We cannot pick and choose which one we would like to use; instead it is totally associated with a particular authentic way of reading with all its rules.
In the riwaayah of Hafs, which the majority of the Muslims in the world recite, it is read only with a , and we cannot read it with a when reciting the Qur'an with the way of Hafs. 
'Afwan