From Wikipedia:
Sujud Sahwi or Sajdah of forgetfulness occurs during the ritual salat prayer. Out of forgetfulness a person can either omit obligatory parts of salat (Qabli) or add to the salat (Ba'adi). In either cases the person corrects his/her salat by doing the Sujud Sahwi.
What must I do as remedy for a Magrib that I observed 4 raka'ats instead of three? After Teslim I noticed I did four Raka'ats?
If you did it without intention, it does not matter. If you intended it, the salah is not acceptable and you have to perform the salah again. If you realize it while rising to the fourth raka'at, return immediately to the previous state and do two sujud sahwi and say "سبحان من لا ينام ولايسهو" in each.
If you did four Rakats instead of three. If you come to remember before finising Prayer ( in last sitting state: Athahiyath) do two Sajdhas after finishing all duas in Athahiyath before giving Salaam. Prophet Muhammed(pbuh) Sajdhas was long from ordinary sajdhaas.
But if you come to know after finishing prayer. Do the same 2 sajdhas and give salaam again. There is no Special duas taught by Prophet Muhammed(pbuh) for the 2 sajdhas. but it was long than ordinary sajdhas.
