Musnad Ahmad 30:2790

Muhammad ibn Ja'far and Abu al-Nadr narrated to us, they said: Shu'bah narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, 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 the prayer. And whoever catches two rak'ahs of the afternoon prayer before the sun sets, he has caught the prayer."