Prayer: Forgetting Impurity During or After Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 790
If someone forgets that his body or clothing is impure and remembers it during or after prayers, in the event that his forgetfulness was due to carelessness and heedlessness, he must perform the prayer again based on obligatory precaution. If the prescribed time for the prayer has expired, he must make it up. [If his forgetfulness was not due to carelessness and heedlessness and] he remembers after prayers [that his body or clothing was impure], it is not necessary for him to perform the prayer again. However, if he remembers this during prayers, he must act according to the instructions that will be mentioned in the next ruling.