What should someone do if they mistakenly give salam in the first sitting, thinking it is the final sitting?

If a person performing a four-unit prayer mistakenly gives salam in the first sitting, thinking it is the final sitting, and has not done anything that invalidates the prayer after this salam—such as speaking or turning away from the qibla—they should continue the prayer from where they left off and perform the prostration of forgetfulness (sehiv secdesi) at the end of the fourth unit. Otherwise, the prayer must be performed again. If the mistake of giving salam in the first sitting is due to ignorance rather than forgetfulness, the prayer must be repeated. For example, if someone who is not a traveler (seferî) performs a prayer that should be four units as only two units, thinking they are a traveler, this prayer must be performed again as four units (İbn Âbidîn, Reddü’l-muhtâr, 2/129).

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu