Answered by Shaykh Faraz Rabbani
Is saying “Amin” after Surah Fatiha a fard or sunnat? Lately I realize that in all my lone prayers, I have unintentionally missed saying “Amin”. Does that invalidate my prayer if I am saying it alone? Does it necessitate forgiveness prostrations (sajda sawhu)?
Walaikum assalam wa rahmatullah,
Saying the amin (quietly) is a sunna: leaving it is improper, but does not affect the validity of one’s prayer. [Nasafi, Kanz al-Daqa’iq; Shurunbulali, Nur al-Idah]
See:
The Rulings of the Sacred Law
http://www.sunnipath.com/resources/Questions/qa00002398.aspx
However, the good in this life and the next lies in following the example of the Beloved of Allah (Allah bless him and give him peace). As such, one should not miss out on these Prophetic sunnas, which give light to one’s worship, life, and soul.
And Allah alone gives success.