Answered by Mawlana Ilyas Patel
What should I do if I don’t remember whether I read sura Fatiha or not, what should I do?
Reciting the Fatiha and a sura or three verses after the opening Takbir is mandatory (wajib). If one does not remember whether one recited the Fatiha or not with reasonable surety, one will have to do the forgetfulness prostration (sajdat al-sahw). [Shurunbulali, Nur al-Idah]
I hope this helps.
[Mawlana] Ilyas Patel
Checked and Approved by Shaykh Faraz Rabbani