Musnad Ahmad 32:468

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Yahya ibn Abi Kathir, from Hafs ibn 'Ubayd Allah ibn Anas, from Anas ibn Malik, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to combine the Dhuhr and 'Asr prayers, and the Maghrib and 'Isha prayers, while traveling."