Sahih Ibn Khuzaymah

Then he should be alerted by saying "Subhanallah" that he forgot to sit for Tashahhud, so he should continue his prayer and, after standing up, should not sit down again, and before saying the Salam, he should perform two prostrations.

Sayyiduna Ibn Buhaynah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) led us in prayer, then narrated the rest of the hadith. Yahya bin Hakim, in his narration, mentions these words: So we reminded the Prophet (peace and blessings be upon him) by saying "Subhanallah," but when he stood up straight, he continued the prayer and did not return (did not sit down). In the narration of Janab Fadl, it is mentioned: They tried to draw the attention of the Prophet (peace and blessings be upon him) by saying "Subhanallah," but he continued the prayer and did not return (to sit).

It is narrated from Sayyiduna Sa’d bin Abi Waqqas (may Allah be pleased with him) that he stood up (instead of sitting for Tashahhud) after two rak’ahs, so the followers tried to draw his attention by saying “Subhanallah.” But he completed the prayer, and then, at the end of the prayer, he performed two prostrations (of forgetfulness), and said: “What did you think, that I would sit down? Indeed, I did as I saw the Messenger of Allah (peace and blessings be upon him) do.” These are the words of the narration of Ibn Mani’. Imam Abu Bakr (may Allah have mercy on him) says: “I think that Abu Mu’awiyah made a mistake in this chain.”