Prayer: Forgetting a Rukn and Performing a Non-Rukn Act
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1096
If a person forgets a rukn and performs the act after it which is not a rukn – for example, before performing two sajdahs he says tashahhud – he must perform the rukn and then perform again what he mistakenly performed before it.