What should one do who forgot the first Tashahhud and the prostrations of forgetfulness (sujood as-sahw), then completed his prayer and said the Tasleem, then remembered?

If you forget the first Tashahhud (definition) while praying behind an Imam, you are not required to perform the prostration of forgetfulness (sujud (definition) sahv) because the Imam bears it on your behalf. However, if you are praying alone, some scholars require you to perform the prostration of forgetfulness before the salaam. If you have already said the salaam and then remember before a long interval has passed, you must perform the prostration. If a long interval has passed, then there is no prostration, and your prayer is valid according to many scholars, though some say it is invalid. The safer option is to repeat the prayer.