Musannaf Ibn Abi Shayba 3:2021

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Yunus, from al-Hasan, regarding a man who forgot a prostration from his prayer and did not remember it until he was in the last rak'ah of his prayer. He said: He performs three prostrations in it. If he does not remember it until he finishes his prayer, provided that he has not yet performed the taslim, he said: He performs one prostration, as long as he has not spoken. If he has spoken, he restarts the prayer.