Prayer: Posture in Nafilah Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 753
Nāfilah prayers can be performed in a sitting position even if one does so voluntarily, and [if someone performs these prayers in a sitting position,] it is not necessary to count two rakʿahs as one rakʿah. However, it is better to perform them in a standing position, except for the nāfilah of ʿishāʾ, which based on obligatory precaution, must be performed in a sitting position.