What is the ruling on increasing the number of rak'ahs in prayer?

If the increase in the number of rak'ahs is intentional, the prayer becomes void and must be repeated, and the one who performed it is sinful. This is except in the case of absolute supererogatory prayers, if the worshiper intended a certain number [of rak'ahs] then changed his intention and increased it. However, if the increase was due to forgetfulness, then the prostration of forgetfulness (sujood al-sahw) is sunnah for him.