Musnad Abu Ya'la al-Mawsili 159:36

Bishr ibn al-Walid narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from his father, from Abd al-Rahman al-A'raj, from Abu Hurayrah, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Whoever catches a prostration of the Afternoon (Asr) prayer before the sun sets, or of the Morning (Subh) prayer before the sun rises, has indeed caught the prayer."