It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Prophet Muhammad (peace and blessings be upon him) said: "Whoever forgets the two Sunnah (rak‘ahs) of Fajr prayer should offer them after the sun has risen."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الركعتين قبل الفجر وما فيها من السنن / 1117