How Should a Person who Mistakenly Sits Down by Assuming that He is in the Last Rakat Complete His Prayer?

How should a person who mistakenly sits down by assuming that he is in the last rakat complete his prayer?

While establishing a fardh or nafl prayer, a person who gives salam after reciting tashahhud at the end of any rakat before the last rakat by thinking that it was the last rakat can stand up immediately and complete the remaining rakats if he did not perform any acts contrary to prayer such as turning his chest away from the qiblah, speaking, and laughing. He performs sajdah al-sahw at the end of prayer and completes the prayer. But if he does an act that invalidates prayer after giving salam although he did not complete the missing rakats, he needs to perform the prayer again (Marghinani, al-Hidayah, II, 81; Ibn Nujaym, al-Bahr, I, 311; Ibn Abidin, Radd al-Muhtar, II, 558-559).

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