What Should We Do When We Give Salam In The First Sitting By Thinking That It Is The Last Sitting?

What should we do when we give salam in the first sitting by thinking that it is the last sitting?

A person who gives salam in the first sitting due to by mistake thinking that he is in the last sitting continues his prayer provided that he did not do any act not related to the prayer such as speaking and turning away from the qiblah, and performs sajdah al-sahw at the end of the fourth rakat. Otherwise, he should establish that prayer again.

If the mistake of giving salam in the first sitting results from lack of information rather than making a mistake, he will need to re-establish it. For example, if a person establishes a four-rakat prayer in two rakats thinking that he is a musafir (traveler) although he is not, he will need to establish the prayer in four rakats again (Ibn Abidin, Radd al-Muhtar, II, 559).

Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWA