Sayyiduna Thabit bin Qays bin Shammas (may Allah be pleased with him) said: I came to the mosque while the Prophet (peace and blessings be upon him) was in prayer. When the Prophet (peace and blessings be upon him) said the salam, he turned towards me; I had begun praying. The Prophet (peace and blessings be upon him) kept looking at me while I continued praying. When I finished, the Prophet (peace and blessings be upon him) asked: "Did you not pray with us?" I said: "Yes." The Prophet (peace and blessings be upon him) said: "Then what prayer was this?" I said: "O Messenger of Allah! These were the two sunnah rak'ahs of Fajr, which I had not been able to pray." The narrator said: The Prophet (peace and blessings be upon him) did not object to it.
Al-Tabarani recorded it; its chain contains two narrators who are not named, and Baqiyyah bin al-Walid reported it from al-Jarrah bin Minhal with 'an'anah (using "from").
The narrator al-Jarrah is munkar al-hadith (his narrations are rejected); this was stated by al-Bukhari (may Allah have mercy on him) and Muslim.