Musannaf Ibn Abi Shayba 35:63

Yazid narrated to us, from Muhammad ibn Ishaq, from Hafs ibn Ubayd Allah ibn Anas, who said: We used to travel with Anas to Makkah. When the sun would pass its zenith while he was at a stopping place, he would not mount until he had prayed Dhuhr. Then, when he set out and the time for Asr arrived, he would pray Asr. If he departed from his stopping place before the sun passed its zenith and the time for prayer arrived, we would say, "The prayer." He would say, "Travel on." Then, when it was between the two prayers, he would stop and combine the Dhuhr and Asr. Then he said: I saw the Prophet, peace and blessings of Allah be upon him, when he would join his morning travel with his afternoon travel, he would do like this.