Sa'id bin Jubayr (may Allah have mercy on him) narrates from Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) combined Zuhr and Asr, and Maghrib and Isha, during the journey of the Battle of Tabuk. I said to Ibn Abbas (may Allah be pleased with them both): What prompted him to do that? He said: So that his (peace and blessings be upon him) Ummah would not be put in hardship.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5549