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) used to combine Zuhr and Asr, and combine Maghrib and Isha, while travelling, without any urgent matter hastening him, or an enemy pursuing him, or fear of anything.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11025