Prayer: Mistaking the First and Second Sajdah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1099
If a person performs the first sajdah thinking that it is the second sajdah, or if he performs the second sajdah thinking that it is the first, his prayer is valid. [The sajdah he deems as] his first sajdah is counted as the first sajdah, and [the sajdah he deems as] his second sajdah is counted as the second sajdah.