What is the ruling on someone who prayed five rak'ahs and no one noticed until some time after the salām, and is his prayer valid?

It is prescribed for whoever adds an extra rak'ah to their prayer by mistake to perform the prostration of forgetfulness (sujud al-sahw). This prostration of forgetfulness should be performed after the salutation (taslim), even if a long time has passed between it and the prayer. The prayer is valid in all cases, even if the worshiper does not perform the prostration of forgetfulness, because the prostration performed after the salutation does not invalidate the prayer by its omission.