From Sahih Muslim (Hadith 1630):
Ibn 'Abbas reported that the Messenger of Allah (ﷺ) 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
Reference: Muslim 1630