Sayyiduna Qays bin Sa'd (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) passed by me while I was praying two rak'ahs after the Subh (Fajr) prayer, and he (peace and blessings be upon him) said: "What prayer is this?" I said: I had not prayed the Sunnah of Fajr, and these are those two rak'ahs. At this, the Messenger of Allah (peace and blessings be upon him) remained silent.
Hadith Referenceالمعجم الكبير للطبراني / باب القاف / 16028