Musnad Abu Ya'la al-Mawsili 159:84

Daud ibn 'Amr al-Dabbi narrated to us, Ibn Abi al-Zinad narrated to us, from his father, from al-A'raj, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever catches a prostration of the Afternoon (Asr) prayer before the sun sets, or the Morning (Subh) prayer before the sun rises, then he has caught the prayer."