Majma al-Zawaid 2919 — Forgetfulness in prayer

Abu Khaldah says: I asked Ibn Sirin: "I pray, and I do not know whether I have prayed two rak'ahs or four." He said: Sayyiduna Abu al-Uryan narrated this hadith to me: One day the Prophet (peace and blessings be upon him) prayed and then went into the house. Among those present was a man who had long hands, whom the Prophet (peace and blessings be upon him) had named Dhu al-Yadayn. Sayyiduna Dhu al-Yadayn said: "O Messenger of Allah! Has the prayer been shortened, or have you forgotten?" The Prophet (peace and blessings be upon him) said: "It has neither been shortened, nor have I forgotten." He said: "No, rather you have perhaps forgotten the prayer." The narrator says: So the Prophet (peace and blessings be upon him) went forward and led them in two rak'ahs, then gave salam, then said the takbir and prostrated like his usual prostrations or perhaps somewhat longer, then said the takbir and raised his head. The narrator Muhammad did not remember whether the narration mentions that he gave salam afterwards or not.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators are the narrators of the Sahih.