It is permissible for the worshipper to build upon what he has already prayed if he forgetfully says the salam after two rak‘ahs before completing the prayer. And the explanation of the evidence that forgetfully saying the salam before finishing the prayer does not invalidate the prayer.

It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) led the prayer and forgot, so he (peace be upon him) ended the prayer with salam after two rak‘ahs. Dhu’l-Yadayn said to him (peace be upon him),“Has the prayer been shortened or did you forget?”So he (peace be upon him) said,“Neither has the prayer been shortened nor did I forget.”Then he (peace be upon him) asked (the companions),“Is it as Dhu’l-Yadayn is saying?”(The companions replied,“Yes.”) So he (peace be upon him) stood up and completed the (remaining) prayer, then performed two prostrations. Imam Abu Bakr (may Allah have mercy on him) says that only Abu Kurayb and Bishr bin Khalid narrate this hadith from Abu Usamah.