Musnad Abu Ya'la al-Mawsili 155:53

'Abd al-A'la ibn Hammad al-Narsi narrated to us, Mu'tamir ibn Sulayman narrated to us, he said: I heard Ma'mar narrating from Ibn Tawus, from his father, from Ibn 'Abbas, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him) that he said: "Whoever catches a rak'ah of the 'Asr prayer before the sun sets, and a rak'ah of the Subh (Fajr) prayer before the sun rises, then he has caught it."