Musannaf Ibn Abi Shaybah

Hazrat Hafs bin Ubaidullah bin Anas says that we used to travel with Hazrat Anas bin Malik (may Allah be pleased with him). When the sun would decline and he was at a stop, he would not mount before offering the Asr prayer. And when he would set out and the time for Asr would come, he would offer the Asr prayer. If he departed from a stop before the sun declined, we would say to him that the time for prayer is about to come. He would say, "Keep moving." Then, when the time between the two prayers would come, he would dismount and offer both the Zuhr and Asr prayers. Then he would say, "I have seen the Messenger of Allah (peace and blessings be upon him) do the same."