Musnad Ahmad 30:2824

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed me, from Zayd ibn Aslam, from Ata ibn Yasar, and from Busr ibn Sa'id, and from al-A'raj, from Abu Hurayrah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever catches a rak'ah of the morning [prayer] before the sun rises, he has caught it. And whoever catches a rak'ah of the afternoon [prayer] before the sun sets, he has caught the afternoon [prayer]."