Musnad Ahmad 32:583

'Abd al-Samad narrated to us, Harb—meaning Ibn Shaddad—narrated to us, Yahya narrated to us, Hafs ibn 'Ubayd Allah ibn Anas narrated to us, that Anas narrated to him: "That the Messenger of Allah (peace and blessings of Allah be upon him) used to combine these two prayers while traveling," meaning the Maghrib and the 'Isha.