Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, we would like to cite the following fatwa published on www.islmaqa.info:
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
If a person mispronounces Al-Fatihah in a way that does not distort the meaning, his prayer is valid, whether he is leading others in prayer or is praying alone.
But with regard to the kind of mispronunciation that distorts the meaning, if the person knows the meaning, such as if he says “Sirat alladheena an`amtu `alayhim [meaning “The way of those on whom I have bestowed my grace”, instead of the correct version an`amta (The way of those on whom You have bestowed Your Grace)], and he knows that this verbal form is wrong, then the prayer is not valid. (Majmu` al-Fatawa, 22/443)
Almighty Allah knows best.