Question:
Do we have to say Ameen whenever we hear Surah Fatiha or is it only limited to Salat?
Answer:
In the name of Allah, Most Compassionate, Most MercifufSalah will be valid even if the thoughts are intentional, however, this will result in reduction of thawab and if the thoughts came unintentionally and you tried to thwart them you will get thawab for your efforts.
A hadith in Muslim says:
“Verily Allah has forgiven my Ummah for the thoughts their minds whisper as long as they don’t practice (on the thougts) or put them in words”