It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him), while resident in Madinah, without any fear and without any necessity, combined the Zuhr and Asr prayers, and also combined the Maghrib and Isha prayers. The people asked: O Abu Abbas! Then why (did he do so)? He said: "He (peace and blessings be upon him) wanted not to put his Ummah in hardship."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12464