It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that Qatadah (may Allah have mercy on him) was asked this question: If a man has prayed only one rak'ah of Fajr and then the sun rises, what is the ruling? He narrated with his chain that the Noble Prophet (peace and blessings be upon him) said: Such a person should complete his prayer.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 10359