Musnad Abu Ya'la al-Mawsili 159:54

Narrated to us Wahb, informed us Khalid, from Abd al-Rahman, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, who said: The Messenger of Allah (peace be upon him) said: "Whoever catches one rak'ah of the dawn (prayer) before the rising of the sun, he has caught the dawn (prayer). And whoever catches two rak'ahs of the afternoon (prayer) before the setting of the sun, he has caught the afternoon (prayer)."