Sahih Muslim Book 4, Hadith Number 1520.

Ibn ‘Abbas reported that the Messenger of Allah (may peace be upon him) combined the noon prayer with the afternoon prayer and the sunset prayer with the ‘Isha prayer in Medina without being in a state of danger or rainfall. And in the Hadith transmitted by Waki’ (the words are): “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.”