Musnad Ahmad 30:680

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, from Ma'mar, from Ibn Tawus, from his father, from Ibn Abbas, who said: "Whoever catches one rak'ah of the Asr prayer before the sun sets, he has caught it." It is narrated from Ibn Abbas, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him: "And whoever catches one rak'ah of the Fajr prayer before the sun rises, he has caught it."