It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) said: Whoever gets one rak'ah of the Fajr prayer before sunrise, he should complete it with the second rak'ah.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 10751