Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1263
Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: He who finds one rak'ah at dawn before the rising of the sun, he in fact finds the dawn prayer. and he who finds one rak'ah of the afternoon prayer before sunset, he in fact finds the afternoon prayer.