Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: When the Messenger of Allah (peace and blessings be upon him) was on a journey and set off at the time the sun passed its zenith, he would combine Zuhr and Asr; and if he set off before that, he would combine them at the time of Asr.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11474