Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1517
Ibn 'Abbas reported that the Messenger of Allah (may peace be upon him) combined the prayers as he set on a journey in the expedition to Tabuk. He combined the noon prayer with the afternoon prayer and the sunset prayer with the 'Isha' prayer. Sa'id (one of the rawis) said to Ibn 'Abbas: What prompted him to do this? He said: He wanted that his Ummah should not be put to (unnecessary) hardship.