Musnad Ahmad 30:1464

Muhammad ibn Ja'far narrated to us, Muhammad ibn 'Amr narrated to us, from Abu Salamah, from Abu Hurairah, from the Prophet, may Allah bless him and grant him peace, that he said: "Whoever catches a rak'ah of the morning prayer before the sun rises, he has caught it. And whoever catches a rak'ah or two rak'ahs of the afternoon prayer before the sun sets, he has caught it."