Musnad Ahmad 30:420

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Abu Salamah ibn 'Abd al-Rahman ibn 'Awf, from Abu Hurayrah, that the Prophet of Allah, peace and blessings of Allah be upon him, said: "Whoever catches a rak'ah of the dawn 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 it."