Musannaf Ibn Abi Shaybah 6597 — The two rakahs of Fajr when one misses

Sayyiduna Qays bin Amr (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) saw a man praying two rak'ahs after Fajr and said to him: "Is the Fajr prayer prayed twice?" The man said: "I had not prayed the sunnah rak'ahs before the obligatory prayer of Fajr, so I am praying them now." On hearing this, the Prophet (peace and blessings be upon him) remained silent.