When one gives the salam after two rakahs, then remembers he has not completed the prayer

Ata said: Sayyiduna Ibn al-Zubayr (may Allah be pleased with him) led the prayer and said the salam after two rak'ahs. Then he went to the Black Stone and touched it. The people said "Subhan Allah", so he came back, completed the prayer and performed two prostrations. Ata said: I mentioned this to Sayyiduna Ibn Abbas (may Allah be pleased with them both), and he said: "How excellent is Ibn al-Zubayr! He did not depart from the Sunnah of his Prophet."

Abu Ubaydah said: Sayyiduna Abdullah (may Allah be pleased with him) said the salam after praying two rak'ahs, then he stood up, completed the prayer and performed two prostrations.

Ibrahim said, concerning a man who makes a lapse in his prayer and says the salam after two rak'ahs: He should continue his prayer and perform the two prostrations of forgetfulness.

Ibn al-Asbahani said: Ibn Abi Layla led us in prayer and said the salam after two rak'ahs. We said "Subhan Allah", so he stood up and completed the prayer. When he had finished the prayer, he performed two prostrations. Ibn al-Asbahani said: I mentioned this to Ikrimah, and he said: "This is a very good way."

Al-Hasan said: When someone says the salam after two rak'ahs, he should complete the prayer and perform the two prostrations of forgetfulness.