Musannaf Ibn Abi Shayba 35:133

Ali ibn Mushir narrated to us, from Muhammad ibn Amr, from Abu Salamah, from Abu Hurairah, from the Prophet (peace be upon him), who said: "Whoever catches a rak'ah of the Asr prayer before the sun sets has caught the prayer. Whoever catches a rak'ah of the Fajr prayer before the sun rises has caught the prayer." And it was mentioned that Abu Hanifah said: "If he prays a rak'ah of the Fajr [prayer] then the sun rises, it does not suffice him."