Prayer: If someone says Bismillah but forgets to intend a sūrah, what should they do?

Practical Laws of Islam as per the teachings of Imam Khamenei

Prayer: If someone says Bismillah but forgets to intend a sūrah, what should they do?

English:

Question #464:

Someone had the intention of reciting the Fātiḥah and Ikhlās chapters at the beginning or is accustomed to reciting them. However, he happened to recite the bismillah and forgot to specify the chapter. Should he intend a specific chapter and then recite the bismillah?

Answer #464:

It is not obligatory for him to repeat the bismillah. Rather, he can consider the bismillah that he already recited sufficient for any chapter he wants to recite afterwards.

-Imam Khamenei, Practical Laws of Islam, Importance and Conditions of Prayer