It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever has not prayed the two rak'ahs of Fajr, let him pray them after the sun has risen."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4555