Musnad Abu Ya'la al-Mawsili 152:345

Abu Bakr ibn Abi Shaybah narrated to us, Bakr ibn Abd al-Rahman narrated to us, Isa narrated to us, from Ibn Abi Layla, from Abu Qays al-Awdi, from Ibn Mas'ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to combine the two prayers while traveling.