Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1519
Mu'adh b. Jabal reported: The Messenger of Allah (may peace be upon him) combined in the expedition to Tabuk the noon prayer with the afternoon prayer and the sunset prayer with the 'Isha' prayer. He (one of the narrators) said: What prompted him to do that? He (Mu'adh) replied that he (the Holy Prophet) wanted that his Ummah should not be put to (unnecessary) hardship.