Complete what you forgot if the time is short

When a worshipper omits one or more rak’ahs of his prayer, then he remembers when he is still in the place where he prayed or in the mosque, and only a short time has elapsed, such as five minutes, then he should complete the prayer by doing what he omitted, then he should say the salaam and do the prostration of forgetfulness, then say the salaam again. But if he does not remember until after a long time has passed, such as half an hour, or a long time after leaving the mosque, then he should repeat the entire prayer and consider the first invalid, because there was no continuity between the rak’ahs.