I prayed Zuhr, and after that I remembered that I had only prayed three rak’ahs. Should I pray the fourth and do sujood al-sahw (prostration of forgetfulness), or should I repeat the whole prayer?


Praise be to Allaah.
 

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.