Sayyiduna Qays bin Amr (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) saw a man praying two rak'ahs after the Fajr prayer. He (peace and blessings be upon him) said: "The Fajr prayer is two rak'ahs." The man said: I had not prayed the two rak'ahs before Fajr, so I am praying those two rak'ahs. He (peace and blessings be upon him) remained silent.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4552