Musnad Ahmad 30:2999

Yahya narrated to us, from Abdullah bin Saeed bin Abi Hind, who said: Abd al-Rahman al-A'raj narrated to us, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever catches a prostration of the morning prayer before the sun rises, he has caught it. And whoever catches a prostration of the afternoon prayer before the sun sets, he has caught it."