Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) combined Zuhr and Asr, and Maghrib and Isha. He (peace and blessings be upon him) was asked about it, and he (peace and blessings be upon him) said: "I did this so that my Ummah would not be put in hardship."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10479