Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): The Prophet (peace and blessings be upon him) said: "When true dawn breaks, then no other (voluntary) prayer is to be performed except the two rak'ahs (sunnah) of Fajr."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Isma'il bin Qays, who is da'if (weak).