Suppose that someone is praying salat of asr. After the the second rakat, he/she forgets that he has another 2 rakats and he finishes it. After he finishes everything and gets going back to his work, he notices that he only did 2 rakats of the salat.
What must he do in this situation? Pray all the 4 rakats from the beginning? Or pray only the remaining 2 rakats over it? Or is there a different solution in the Sunnah?