Sahih Muslim Book 6, Hadith 63
The Messenger of Allah (ﷺ) 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.
We set out with the Messenger of Allah (ﷺ) on the Tabuk expedition, and he observed the noon and afternoon prayers together and the sunset and 'Isha' prayers together.
" I said to Ibn 'Abbas: What prompted him to do that? He said: So that his (Prophet's) Ummah should not be put to (unnecessary) hardship." And in the hadith transmitted by Mu'awiya (the words are):" It was said to Ibn 'Abbas: What did he intend thereby? He said he wanted that his Ummah should not be put to unnecessary hardship."